Wildcat gatecrashing a navy function? (6)
I believe the answer is:
arctan
'function?' is the definition.
(I know that arctan is a type of circular function)
'wildcat gatecrashing a navy' is the wordplay.
'wildcat' becomes 'cta' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'gatecrashing' means one lot of letters goes inside another.
'navy' becomes 'RN' (abbreviation for Royal Navy).
'a'+'rn'='arn'
'cta' inserted inside 'arn' is 'ARCTAN'.
(Another definition for arctan that I've seen is " Function".)