Dull prisoner concealing a nail, oddly (9)
I believe the answer is:
inanimate
'dull' is the definition.
(I've seen this before)
'prisoner concealing a nail oddly' is the wordplay.
'prisoner' becomes 'inmate' (inmate can mean someone in prison).
'concealing' means one lot of letters goes inside another.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
The alternate letters of 'nail' are 'ni'.
'a'+'ni'='ani'
'inmate' placed around 'ani' is 'INANIMATE'.
(Other definitions for inanimate that I've seen before include "Showing no feeling" , "Showing no sign of life" , "Dull" , "lacking in spirit" , "spiritless" .)