Stress as adult's missed coach (6)
I believe the answer is:
strain
'stress' is the definition.
(synonyms)
'as adult's missed coach' is the wordplay.
'adult' becomes 'a' (abbreviation).
'missed' is a deletion indicator.
'coach' becomes 'train' (both can mean to instruct a sportsperson).
'as' with 'a' removed is 's'.
's'+'train'='STRAIN'
(Other definitions for strain that I've seen before include "Blood" , "Weaken -- sieve" , "Force - melody" , "Race" , "making a great demand on the emotions?" .)