Barrel I carried containing heirloom (5)
I believe the answer is:
relic
'heirloom' is the definition.
(I've seen this in another clue)
'barrel i carried containing' is the wordplay.
'containing' says the answer is hidden in the clue.
'RELIC' is hidden within 'barrel i carried'.
(Other definitions for relic that I've seen before include "Memorial object" , "Reminder of times past" , "One interestingly old" , "Object of historical interest" , "E,g. dead saint's bone" .)