Over time gathering ten tonnes (5)
I believe the answer is:
extra
'over' is the definition.
(extra can mean left over)
'time gathering ten tonnes' is the wordplay.
'time' becomes 'era' (era is a kind of time).
'gathering' is an insertion indicator.
'ten' becomes 'X' (Roman numeral).
'tonnes' becomes 't' (abbreviation).
'x'+'t'='xt'
'era' going around 'xt' is 'EXTRA'.
(Other definitions for extra that I've seen before include "A bit on the side" , "Ancillary" , "one added to the score" , "Additional or supplementary" , "Actor in non-speaking role" .)