I complain about Ruth’s mother-in-law (5)
I believe the answer is:
naomi
'ruth's mother-in-law' is the definition.
(biblical figure)
'i complain about' is the wordplay.
'complain' becomes 'moan' (moan can mean to complain or whinge).
'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 "Ruth's mother-in-law (OT)" , "Girl's name -- I moan (anag)" , "OT female" , "woman in the Bible" , "woman's name" .)