Boat race crew discovered American pistol at turning (7)
I believe the answer is:
regatta
'boat race' is the definition.
(I've seen this in another clue)
'crew discovered american pistol at turning' is the wordplay.
'discovered' indicates the central letters (dis-covered - outer letters removed).
'american pistol' becomes 'gat' (informal US term).
'turning' says the letters should be written in reverse.
The middle of 'crew' is 're'.
'at' back-to-front is 'ta'.
're'+'gat'+'ta'='REGATTA'
(Other definitions for regatta that I've seen before include "Yachting festival" , "Race meeting for boats" , "which may involve eights" , "Boat race meeting" , "Boat festival" .)