It messes around with a man or a lover (9)
I believe the answer is:
inamorata
'lover' is the definition.
(I've seen this in another clue)
'it messes around with a man or a' is the wordplay.
'messes around with' indicates anagramming the letters (I've seen 'messing around' mean this).
'a'+'man'+'or'='amanor'
'it' with its letters anagrammed with 'amanor' can make 'inamorat'.
'inamorat'+'a'='INAMORATA'
(Other definitions for inamorata that I've seen before include "Female lover" , "loved one" , "6 across [BOSOM] friend" , "Sweetheart" , "Woman in love" .)