"My word!" (7)
'my word' is the definition.
(to give someone your word is to make a promise)
This is the entire clue.
(Other definitions for promise that I've seen before include "Assurance" , "Cause for hope" , "Oath" , "Verbal or other commitment of one person to another" , "Vow" .)
'word' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I can't explain the remainder of the clue.
(Other definitions for heavens that I've seen before include "Places of supreme happiness" , "Exclamation of surprise" , "Exclamation (Good ____ !)" , "Save hen (anag.)" , "Gosh!" .)
(Other definitions for by jove that I've seen before include "Crikey!" , "Good gracious!" , "Well, I'm blowed!" , "Old exclamation of surprise invoking Jupiter" , "Well, I'll be blowed!" .)
(Other definitions for gee that I've seen before include "Word of encouragement to a horse" , "American exclamation" , "Mild expression of surprise, may go before whiz" , "Horse - surprised expression" , "Exclamation of surprise" .)
'my word' is the definition.
This is all the clue.
(Other definitions for i say that I've seen before include "Expression of surprise" , "Gosh!" , "Good heavens!" , "Look here; oh, my" , "Blimey!" .)
(Other definitions for by george that I've seen before include "Anne succeeded so" , "Dated expression of surprise" , "Well I never" , "planes might be flown thus?" , "Animal Farm was -- ----- -?" .)
(Other definitions for by gum that I've seen before include "Brother in N England" , "Mild oath (how things can be fixed!)" , "It's emphatically uttered" .)