He's very happy to linger topless beside lake (5)
I believe the answer is:
larry
'he's very happy to linger' is the definition.
The definition suggests a singular noun which matches the answer.
'topless beside lake' is the wordplay.
'topless' becomes 'arry' (I can't explain this - if you can you should give a lot more credence to this answer).
'beside' means one lot of letters go next to another (I've seen this in other clues).
'lake' becomes 'L' (geographical abbreviation).
'arry' put after 'l' is 'LARRY'.
Can you help me to learn more?
(Other definitions for larry that I've seen before include "happy man" , "Proverbially happy fellow" .)