United force American-English defender at the outset (5)
I believe the answer is:
fused
'united' is the definition.
(similar in meaning)
'force american-english defender at the outset' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'american' becomes 'us' (abbreviation for United States).
'at the outset' indicates taking the first letters.
The initial letters of 'english defender' is 'ed'.
'f'+'us'+'ed'='FUSED'
(Other definitions for fused that I've seen before include "Married" , "Coalesced" , "Combined" , "Equipped with safety electrical device; molten" , "Joined by melting" .)