One stealing fish is a cheeky youngster (14)
I believe the answer is:
whippersnapper
'a cheeky youngster' is the definition.
(I've seen this before)
'one stealing fish' is the wordplay.
'one stealing' becomes 'whipper' (I can't justify this - if you can you should give a lot more credence to this answer).
'fish' becomes 'snapper' (snapper is a kind of fish).
'whipper'+'snapper'='WHIPPERSNAPPER'
'is' is the link.
Can you help me to learn more?
(Other definitions for whippersnapper that I've seen before include "Impudent lad" , "Scallywag" , "Pipsqueak" , "Nonentity" , "cheeky young lad" .)