Astound enthusiasts heading west (4)
I believe the answer is:
stun
'astound' is the definition.
(I've seen this in another clue)
'enthusiasts heading west' is the wordplay.
'enthusiasts' becomes 'nuts' (nut can mean an enthusiast or fan).
'heading west' is a reversal indicator.
'nuts' backwards is 'STUN'.
(Other definitions for stun that I've seen before include "Surprise greatly or daze" , "Render unconscious" , "Nuts (anag)" , "floor" , "Render semi-conscious with blow" .)