In good shape, you can have a bit of fun with it (3)
I believe the answer is:
fit
'in good shape' is the definition.
(I've seen this in another clue)
'bit of fun with it' is the wordplay.
'bit of' says to take the initial letters.
'with' means one lot of letters go next to another.
The initial letter of 'fun' is 'f'.
'f'+'it'='FIT'
'you can have a' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for fit that I've seen before include "Healthy; conform to" , "In trim" , "Apt" , "Suitable; in good health" , "Healthy; spasm" .)