Box, perhaps containing old chief's foot (7)
I believe the answer is:
trochee
'foot' is the definition.
(trochee is a kind of foot)
'box perhaps containing old chief's' is the wordplay.
'box perhaps' becomes 'tree' (box tree is a type of tree).
'containing' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'chief' becomes 'ch' (I've seen this in other clues).
'o'+'ch'='och'
'tree' placed around 'och' is 'TROCHEE'.
(Other definitions for trochee that I've seen before include "service, for example" , "Foot of a stressed and an unstressed syllable" , "A foot of two syllables, long then short" , "Long-short foot (metre)" , "Using foot" .)