Impish youngster having most of shellfish (5)
I believe the answer is:
scamp
'impish youngster' is the definition.
(I've seen this before)
'most of shellfish' is the wordplay.
'most of' means to remove the last letter.
'shellfish' becomes 'scampi' (I've seen this before).
'scampi' with its final letter removed is 'SCAMP'.
'having' is the link.
(Other definitions for scamp that I've seen before include "villain" , "Mischievous child (colloq.)" , "little monkey" , "Little Devil" , "Scallywag" .)