Motivation provided by setter's favourite American (7)
I believe the answer is:
impetus
'motivation' is the definition.
(thesaurus)
'by setter's favourite american' is the wordplay.
'by' means one lot of letters go next to another.
'setter's' becomes 'im' (I've seen this in other clues**).
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
'american' becomes 'us'.
'im' next to 'pet' is 'impet'.
'impet'+'us'='IMPETUS'
'provided' is the link.
(Other definitions for impetus that I've seen before include "Momentum, stimulus" , "Motivation" , "influence" , "Thrust" , "Incentive, stimulus" .)