Maintains trendy poses, overwhelming son (7)
I believe the answer is:
insists
'maintains' is the definition.
(I've seen this in another clue)
'trendy poses overwhelming son' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'poses' becomes 'sits' ('sit' can be a synonym of 'pose').
'overwhelming' indicates putting letters inside.
'son' becomes 's' (genealogical abbreviation for son).
'sits' enclosing 's' is 'sists'.
'in'+'sists'='INSISTS'
(Other definitions for insists that I've seen before include "Demands firmly, not accepting refusal" , "Maintains persistently" , "Maintains firmly" , "Contends" , "Brooks no refusal" .)