Girl I grumble about (5)
I believe the answer is:
naomi
'girl' is the definition.
'i grumble about' is the wordplay.
'grumble' becomes 'moan' (I've seen this before).
'about' shows that the letters should be reversed in order.
'i'+'moan'='imoan'
'imoan' reversed gives 'NAOMI'.
(Other definitions for naomi that I've seen before include "Do I moan about Biblical Ruth's mother-in-law?" , "Ruth's mother-in-law (OT)" , "OT female" , "Biblical woman" , "Girl" .)