Ancient yellow head mislaid (5)
I believe the answer is:
olden
'ancient' is the definition.
(I've seen this before)
'yellow head mislaid' is the wordplay.
'yellow' becomes 'golden'.
'head mislaid' means to remove the first letter.
'golden' with its initial letter taken off is 'OLDEN'.
(Other definitions for olden that I've seen before include "as in time gone by" , "(Of days) in the long distant past!" , "times gone by" , "Led on (anag)" , "of ancient history?" .)