Promise made by a leader of scouts? Certainly! (6)
I believe the answer is:
assure
'promise' is the definition.
(I know that promise can be written as assure)
'a leader of scouts? certainly' is the wordplay.
'leader of' indicates taking the first letters.
'certainly' becomes 'sure' (synonyms).
The initial letter of 'scouts' is 's'.
'a'+'s'+'sure'='ASSURE'
'made by' is the link.
(Other definitions for assure that I've seen before include "Convince, promise" , "Say positively" , "Make sure, guarantee" , "Cover by life insurance" , "Declare confidently, promise" .)