Top adman embodies writer's block (3)
I believe the answer is:
pad
'writer's block' is the definition.
(I've seen this before)
'top adman embodies' is the wordplay.
'embodies' indicates the answer is hidden within the clue.
'PAD' can be found hidden inside 'top adman'.
(Other definitions for pad that I've seen before include "Living quarters, colloquially" , "Writing block" , "writer's block" , "Protective guard; home" , "Cushion" .)