Could he possibly keep a gang leader in line? (5)
I believe the answer is:
nigel
'could he' is the definition.
The definition suggests a singular noun which matches the answer.
'keep a gang leader in line?' is the wordplay.
'keep' becomes 'nie' (I can't explain this - if you can you should give a lot more credence to this answer).
'leader' suggests taking the first letters.
'in' indicates putting letters inside.
'line?' becomes 'l' (used when specifying particular lines from a poem).
The initial letter of 'gang' is 'g'.
'nie' enclosing 'g' is 'nige'.
'nige'+'l'='NIGEL'
'possibly' acts as a link.
Can you help me to learn more?
(Other definitions for nigel that I've seen before include "Fellows sometimes called this" , "We all sign on (anag) -- politician" , "boy" , "-- Kennedy, violinist" , "Chap" .)