Poet, younger still (10)
I believe the answer is:
motionless
'still' is the definition.
(I know that still can be written as motionless)
'poet younger' is the wordplay.
'poet' becomes 'motion' (former Poet Laureate Andrew Motion).
'younger' becomes 'less'.
'motion'+'less'='MOTIONLESS'
(Other definitions for motionless that I've seen before include "Immobile" , "not having anything to vote about?" , "Static" , "Still, unmoving" , "constipated?" .)