Air in small coach (6)
I believe the answer is:
strain
'air' is the definition.
(I know that air can be written as strain)
'small coach' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'coach' becomes 'train' (both can mean to instruct a sportsperson).
's'+'train'='STRAIN'
'in' is the link.
(Other definitions for strain that I've seen before include "Screen" , "Injure (a muscle); distinct breed" , "Pull (a muscle?)" , "Filter - intense effort" , "Family - tension" .)