Stamp stuck to envelope portrayed heads (4)
I believe the answer is:
step
'stamp' is the definition.
(thesaurus)
'stuck to envelope portrayed heads' is the wordplay.
'heads' says to take the initial letters.
The initial letters of 'stuck to envelope portrayed' is 'STEP'.
(Other definitions for step that I've seen before include "Pace (taken one at a time?)" , "Bit of mashed potato maybe" , "Gradation" , "Pace for a pest" , "'Stage, rung (4)'" .)