Groom starts to panic, really envisaging escaping nuptials (5)
I believe the answer is:
preen
'groom' is the definition.
(a bird grooms itself by preening feathers)
'starts to panic really envisaging escaping nuptials' is the wordplay.
'starts to' says to take the initial letters.
The first letters of 'panic really envisaging escaping nuptials' is 'PREEN'.
(Other definitions for preen that I've seen before include "Congratulate (oneself); attend to dress" , "Tidy and clean (plumage)" , "Beautify" , "Dress with elaborate care" , "to give himself a pat on the back" .)