First murderer to collar fit officer (7)
I believe the answer is:
captain
'officer' is the definition.
(I know that captain is a type of airplane pilot)
'first murderer to collar fit' is the wordplay.
I cannot really see how this works, but
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is found within the answer.
'fit' could be 'apt' (I've seen this before) and 'apt' is found in the answer.
The remaining letters 'can' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for captain that I've seen before include "American police officer" , "Commander of a ship" , "One traditionally goes down with a sinking ship" , "Chief" , "Skipper" .)