Granny perhaps seen reversing in Milton Keynes (4)
I believe the answer is:
knot
'granny perhaps' is the definition.
(granny is a kind of knot)
'reversing in milton keynes' is the wordplay.
'reversing' says the letters should be written backwards.
'in' indicates a hidden word.
'tonk' is hidden within 'milton keynes'.
'tonk' back-to-front is 'KNOT'.
'seen' acts as a link.
(Other definitions for knot that I've seen before include "binder?" , "Fastening; sea bird" , "Bend - one nautical mile per hour" , "Fastening such as reef or granny" , "Air mile" .)