Insect devours posh relative (4)
I believe the answer is:
aunt
'relative' is the definition.
(aunt is a type of relative)
'insect devours posh' is the wordplay.
'insect' becomes 'ant' (ant is a kind of insect).
'devours' is an insertion indicator (inserted letters are devoured).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'ant' going around 'u' is 'AUNT'.
(Other definitions for aunt that I've seen before include "Blood relative" , "Female relative" , "Tuna for the odd relative" , "Tuna gone bad for the relative" , "Put up with" .)