Knock senseless in a most unlikely way (4)
I believe the answer is:
stun
'knock senseless' is the definition.
(I've seen this in another clue)
'in a most unlikely way' is the wordplay.
'in' indicates the answer is hidden within the clue.
'STUN' is hidden within 'most unlikely way'.
(Other definitions for stun that I've seen before include "Daze, overwhelm" , "Render unconscious" , "Knock unconscious" , "Put out for the count" , "Concuss; amaze" .)