Souvenir one found in contents of barrel chest (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Hidden answer (contents of . . .) [bar] REL C [hest], with I (one in Roman numerals) inserted. Hidden answer clues normally provide the complete answer, not something that isn’t actually a word and needs further addition to give the answer; perhaps we’re supposed to treat the I as the “contents” of BARREL CHEST (i.e. between the two words) and then take the answer from that?.