Persevere as small, ancient metal boxes start to explode (7,2)
I believe the answer is:
soldier on
'persevere' is the definition.
(I've seen this before)
'small ancient metal boxes start to explode' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ancient' becomes 'old' ('old' can be similar in meaning to 'ancient').
'metal' becomes 'iron' (iron is a kind of metal**).
'boxes' indicates putting letters inside (the outside letters must 'box' inside letters in).
'start to' says to take the initial letters.
The first letter of 'explode' is 'e'.
'iron' going around 'e' is 'ieron'.
's'+'old'+'ieron'='SOLDIER ON'
'as' acts as a link.
(Other definitions for soldier on that I've seen before include "keep going" , "plug away" , "Persist in spite of difficulties" , "Doggedly continue" , "Don't give up" .)