Crawler looks after posh relative (4)
I believe the answer is:
aunt
'relative' is the definition.
(aunt is a kind of relative)
'crawler looks after posh' is the wordplay.
'crawler' becomes 'ant' (an ant crawls).
'looks after' means one lot of letters goes inside another.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'ant' enclosing 'u' is 'AUNT'.
(Other definitions for aunt that I've seen before include "kinswoman" , "Parent's female sibling" , "Sister of father/mother" , "Tuna gone bad for the relative" , "Uncle's wife" .)