Momentum of this writer’s dog, perhaps (American) (7)
I believe the answer is:
impetus
'momentum' is the definition.
(thesaurus)
'this writer's dog perhaps american' is the wordplay.
'this writer's' becomes 'im' (the person writing the crossword clues).
'dog perhaps' becomes 'pet'.
'american' becomes 'us'.
'im'+'pet'+'us'='IMPETUS'
'of' is the link.
(Other definitions for impetus that I've seen before include "Thrust" , "Catalyst" , "Drive" , "Moving force" , "Imputes to force with which a body moves" .)