Wild time with a fatheaded swimmer (5)
I believe the answer is:
fling
'wild time' is the definition.
'fling' can be an answer for 'time' (I have seen 'good time' mean 'fling' so perhaps 'time' could also mean 'fling'). I am unsure of the 'wild' bit.
'with a fatheaded swimmer' is the wordplay.
'with' says to put letters next to each other.
'a fatheaded' becomes 'f' (this could be a standard abbreviation which I don't know about).
'swimmer' becomes 'ling' (ling is a type of fish).
'f'+'ling' is 'FLING'.
Can you help me to learn more?
(Other definitions for fling that I've seen before include "quick one?" , "Brief relationship" , "knees-up at 8 [HIGHLAND GAMES]" , "Throw - short affair" , "Hurl" .)