He is in full clothing while doing it (2,3,3)
I believe the answer is:
in the act
'he is in full clothing while doing it' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'he is in full clothing while doing it' is the wordplay.
I cannot really see how this works, but
'he' is present in the answer.
'in' is within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
The remaining letters 'act' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for in the act that I've seen is " with trousers down?".)