In army officer's wake, I join up with good humour (8)
I believe the answer is:
genially
'with good humour' is the definition.
'in army officer's wake i join up' is the wordplay.
I cannot really understand how this works, but
'i' could be 'a' and 'a' is located in the answer.
'officer' could be 'gen' (abbreviation for general) and 'gen' is present in the answer.
The remaining letters 'illy' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for genially that I've seen before include "In friendly and cheerful manner" , "in affable manner" .)