Fossilised resin from Thatcham, Berks (5)
I believe the answer is:
amber
'fossilised resin' is the definition.
(I've seen this before)
'from thatcham berks' is the wordplay.
'from' says the answer is hidden in the clue.
'AMBER' can be found hidden inside 'thatcham berks'.
(Other definitions for amber that I've seen before include "Honey-yellow colour" , "A yellowy resin" , "Yellow, translucent fossil resin" , "first light" , "Girl" .)