Endlessly regret joke about speed merchant’s approach (3-2)
I believe the answer is:
run-up
'speed merchant's approach' is the definition.
(approach taken by a speed-merchant in cricket)
'endlessly regret joke about' is the wordplay.
'endlessly' means to remove the last letter.
'regret' becomes 'rue' (synonyms).
'joke' becomes 'pun' (punning is a kind of joking).
'about' says the letters should be written backwards.
'pun' in reverse letter order is 'nup'.
'rue' with its last letter taken off is 'ru'.
'ru'+'nup'='RUN-UP'
(Other definitions for run-up that I've seen before include "Approach - preliminaries" , "Preliminary period" , "Cricketing approach?" , "Bowler's approach?" .)