He's in jail again! (3)
I believe the answer is:
lag
'he's' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'in jail again' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LAG' can be found hidden inside 'jail again'.
Can you help me to learn more?
(Other definitions for lag that I've seen before include "Protect from cold" , "Fall behind or cover to prevent heat loss" , "Drag one's feet" , "Go too slow, fail to keep up" , "Fall behind with a convict" .)