Be conscious of payment that's left behind (4)
I believe the answer is:
feel
'be conscious of' is the definition.
(thesaurus)
'payment that's left behind' is the wordplay.
'payment' becomes 'fee' (payment for a job).
'that's left' becomes 'l' (common abbreviation).
'behind' says to put letters next to each other.
'fee'+'l'='FEEL'
(Other definitions for feel that I've seen before include "Be aware of - texture" , "Perceive, sense" , "16A [JUDGE]" , "Find by groping" , "impression" .)