Shooting pain's beginning in gardening activity (8)
I believe the answer is:
speeding
'shooting' is the definition.
'pain's beginning in gardening activity' is the wordplay.
'beginning' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'gardening activity' becomes 'seeding'.
The initial letter of 'pains' is 'p'.
'p' inserted within 'seeding' is 'SPEEDING'.
(Other definitions for speeding that I've seen before include "Travelling too fast" , "Travelling over the limit" , "Travelling rapidly" , "Going too fast" , "Moving very fast" .)