American in Punch sequence (5)
I believe the answer is:
chain
'sequence' is the definition.
(I've seen this before)
'american in punch' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'in' is an insertion indicator.
'punch' becomes 'chin' (slang term meaning to punch someone on the chin).
'a' inserted within 'chin' is 'CHAIN'.
(Other definitions for chain that I've seen before include "trading group" , "A measure" , "unwelcome to house seeker" , "It's as strong as its weakest link" , "British biochemist identified?" .)