Pen centre of stencil grey (6)
I believe the answer is:
encage
'pen' is the definition.
(I've seen this before)
'centre of stencil grey' is the wordplay.
'centre of' means to look at the middle letters.
'grey' becomes 'age' (I've seen this in another clue).
The central letters of 'stencil' are 'enc'.
'enc'+'age'='ENCAGE'
(Other definitions for encage that I've seen before include "Bang up" , "Put behind bars" , "Pen" .)