A pain! He is after power (4)
I believe the answer is:
ache
'a pain' is the definition.
(ache is a kind of pain)
'he is after power' is the wordplay.
'is after' says to put letters next to each other.
'power' becomes 'AC' (AC is an example).
'he' put after 'ac' is 'ACHE'.
(Other definitions for ache that I've seen before include "suffering" , "agony" , "Feel pain" , "Dull continuous pain" , "Hurt; yearn" .)