Pretentious, like an American’s trousers? (5-5)

'pretentious' is the definition.
(fancy-pants can informally mean pretentious)

'an american's trousers?' is the wordplay.
'an american's' becomes 'fancy' (I can't explain this - if you can you should give a lot more credence to this answer).
'trousers?' becomes 'pants' ('pants' can be a synonym of 'trousers').
'fancy'+'pants'='FANCY-PANTS'

'like' acts as a link.