Like invigorating air beginning to boost activity at Epsom? (7)
I believe the answer is:
bracing
'like invigorating air' is the definition.
(something bracing is refreshing or energising)
'beginning to boost activity at epsom?' is the wordplay.
'beginning to' suggests taking the first letters.
'activity at epsom?' becomes 'racing' (Epsom is a race course).
The initial letter of 'boost' is 'b'.
'b'+'racing'='BRACING'
(Other definitions for bracing that I've seen before include "Supporting" , "Imparting vitality" , "form of support" , "Cold" , "brisk" .)