Conjecture the other ranks verify finally (6)
I believe the answer is:
theory
'conjecture' is the definition.
(thesaurus)
'the other ranks verify finally' is the wordplay.
'other ranks' becomes 'or' (military abbreviation).
'finally' says to take the final letters.
The last letter of 'verify' is 'y'.
'the'+'or'+'y'='THEORY'
(Other definitions for theory that I've seen before include "Scientific conjecture" , "Explanation or system of anything" , "Series of ideas seeking to explain something" , "Speculative view" , "The presumption" .)