Make a deep impression in Bletchley (4)
I believe the answer is:
etch
'make a deep impression' is the definition.
(I've seen this before)
'in bletchley' is the wordplay.
'in' indicates a hidden word.
'ETCH' is hidden within 'bletchley'.
(Other definitions for etch that I've seen before include "Eat away with acid" , "Engrave deeply" , "Chase" , "Carve (design) on a surface" , "Wear away by chemical reaction" .)