Force American husband to enthuse (4)
I believe the answer is:
gush
'to enthuse' is the definition.
(I've seen this before)
'force american husband' is the wordplay.
'force' becomes 'G' (short for gravity).
'american' becomes 'US' (abbreviation for United States).
'husband' becomes 'h' (abbreviation).
'g'+'us'+'h'='GUSH'
(Other definitions for gush that I've seen before include "Speak effusively" , "Pour forth" , "Spout powerfully" , "Spout like geyser" , "Be over-effusive" .)