Support firm in housing American relative (6,6)
I believe the answer is:
second cousin
'relative' is the definition.
(second cousin is a kind of relative)
'support firm in housing american' is the wordplay.
'support' becomes 'second' (I've seen this before**).
'firm' becomes 'co' (a firm is a company).
'housing' is an insertion indicator.
'american' becomes 'us'.
'co'+'in'='coin'
'coin' placed around 'us' is 'cousin'.
'second'+'cousin'='SECOND COUSIN'
(Other definitions for second cousin that I've seen before include "Relative" , "Fairly distant relation" , "Some family" .)