Silly fellow's about to start needling the jailer (7)
I believe the answer is:
turnkey
'the jailer' is the definition.
('turnkey' can be a synonym of 'jailer')
'silly fellow's about to start' is the wordplay.
'silly fellow's' becomes 'key' (I can't justify this - if you can you should believe this answer much more).
'about' means one lot of letters go next to another.
'to start' becomes 'turn' (I've seen this before).
'key' put after 'turn' is 'TURNKEY'.
'needling' is the link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for turnkey that I've seen before include "Warder banging up" , "Jailer (arch.)" , "Designed and ready for immediate use by the purchaser" , "Jailer (archaic), (of service) provided ready for use" , "locker" .)