Drive yours truly's sweetheart and American (7)
I believe the answer is:
impetus
'drive' is the definition.
(impetus is a drive to do something)
'yours truly's sweetheart and american' is the wordplay.
'yours truly's' becomes 'I'm' (I'm).
'sweetheart' becomes 'pet' (both are terms of endearment).
'and' says to put letters next to each other.
'american' becomes 'us' (abbreviation for United States).
'im'+'pet'+'us'='IMPETUS'
(Other definitions for impetus that I've seen before include "Drive" , "Force that moves something along" , "Motivation" , "Incentive, stimulus" , "influence" .)