Has clocked stunner in hat (5)
I believe the answer is:
shako
'hat' is the definition.
(shako is a kind of hat)
'has clocked stunner' is the wordplay.
I cannot really understand how this works, but
an anagram of 'has' is 'sha' which is located in the answer.
'stunner' could be 'ko' (a knock-out blow or KO stuns a boxer) and 'ko' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for shako that I've seen before include "old military item" , "Old headgear" , "Capital cover" , "Ceremonial hat" , "military headdress" .)