Leave an impression after really top gag (5)
I believe the answer is:
retch
'gag' is the definition.
(I've seen this before)
'leave an impression after really top' is the wordplay.
'leave an impression' becomes 'etch'.
'after' means one lot of letters go next to another.
'top' indicates taking the first letters.
The first letter of 'really' is 'r'.
'etch' put after 'r' is 'RETCH'.
(Other definitions for retch that I've seen before include "Heave (as if to vomit)" , "Sound and seem to vomit" , "Try to be sick" , "Gag, near-vomit" , "Reach" .)