Fossilised resin found in Cookham, Berkshire (5)
I believe the answer is:
amber
'fossilised resin' is the definition.
(I've seen this before)
'in cookham berkshire' is the wordplay.
'in' says the answer is hidden in the clue.
'AMBER' can be found hidden inside 'cookham berkshire'.
'found' acts as a link.
(Other definitions for amber that I've seen before include "Middle light" , "Ornamental fossil resin" , "The traffic-light between red and green" , "Yellowish-brown colour or resin" , "Yellow-brown colour as of bream" .)