It might have slipped from American leaving field event (4)
I believe the answer is:
disc
'it might have slipped' is the definition.
The definition suggests a singular noun which matches the answer.
'american leaving field event' is the wordplay.
'american' becomes 'US' (abbreviation for United States).
'leaving' is a deletion indicator.
'field event' becomes 'discus' (discus is a kind of field event).
'discus' with 'us' taken away is 'DISC'.
'from' is the link.
Can you help me to learn more?
(Other definitions for disc that I've seen before include "45, eg?" , "Protein-constituent acid" , "Thin circular plate, as in CD" , "In Olympic event" , "jockey may be after" .)