Influence dismissed by heartless colonel (5)
I believe the answer is:
clout
'influence' is the definition.
(thesaurus)
'dismissed by heartless colonel' is the wordplay.
'dismissed' becomes 'out' (I've seen this before**).
'by' means one lot of letters go next to another.
'heartless' means to remove the middle letters.
'colonel' with its centre removed is 'cl'.
'out' after 'cl' is 'CLOUT'.
(Other definitions for clout that I've seen before include "A blow; powerful influence" , "Wallop - influence" , "Lay one on" , "Ne'er cast a ... till May be out, it's said" , "Hit - influence" .)