Quantity in grass picked up (7)
I believe the answer is:
resumed
'picked up' is the definition.
(to pick up is to carry on doing something)
'quantity in grass' is the wordplay.
'quantity' becomes 'sum' (sum is a kind of quantity).
'in' means one lot of letters goes inside another.
'grass' becomes 'reed' (type of grass plant).
'sum' put inside 'reed' is 'RESUMED'.
(Other definitions for resumed that I've seen before include "Recommenced" , "came into play again" , "Carried on again" , "Started again where one had left off" , "Restarted" .)