I complain about Ruth's mother-in-law (5)
I believe the answer is:
naomi
'ruth's mother-in-law' is the definition.
(I've seen this before)
'i complain about' is the wordplay.
'complain' becomes 'moan' (I've seen this before).
'about' says the letters should be written backwards.
'i'+'moan'='imoan'
'imoan' back-to-front is 'NAOMI'.
(Other definitions for naomi that I've seen before include "- - Campbell, supermodel" , "Biblical mother-in-law of Ruth" , "Ruth's mother-in-law (Old Testament)" , "Girl's name, meaning "pleasant"" , "-- Campbell, British model" .)