Jack aged so unexpectedly (3,3)
I believe the answer is:
sea dog
'jack' is the definition.
(thesaurus)
'aged so unexpectedly' is the wordplay.
'unexpectedly' is an anagram indicator.
'aged'+'so'='agedso'
'agedso' with letters rearranged gives 'SEA-DOG'.
(Other definitions for sea dog that I've seen before include "Dosage (anag)" , "Experienced sailor (colloq.)" , "Old tar" , "Experienced mariner" , "mythical creature" .)