One has a lot of fun with it (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
'a lot of fun with it' is the wordplay.
'a lot of' means to remove the first letter.
'with' says to put letters next to each other.
'fun' with its first letter taken away is 'un'.
'un'+'it'='UNIT'
'has' acts as a link.
(Other definitions for unit that I've seen before include "Single thing or person" , "Some fighting men" , "Single item; one" , "squad" , "Single element; one" .)