Wildcat gatecrashing a navy function? (6)
I believe the answer is:
'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).
'cta' inserted inside 'arn' is 'ARCTAN'.
(Another definition for arctan that I've seen is " Function".)