Local fellow hides a youngster (4)
I believe the answer is:
calf
'a youngster' is the definition.
(I've seen this before)
'local fellow hides' is the wordplay.
'hides' indicates a hidden word.
'CALF' can be found hidden inside 'local fellow'.
(Other definitions for calf that I've seen before include "Young animal; part of body" , "Is it above the ankle" , "Bit of leg" , "Young giraffe - piece of iceberg" , "Bovine youngster" .)