Provide way to support dyke-builder from the south (6)
I believe the answer is:
afford
'provide' is the definition.
(affording is a kind of providing)
'way to support dyke-builder from the south' is the wordplay.
'way' becomes 'rd' (short for road).
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'dyke builder' becomes 'offa' (I've seen this before).
'from the south' is a reversal indicator (in a down clue, letters go up).
'offa' back-to-front is 'affo'.
'rd' put after 'affo' is 'AFFORD'.
(Other definitions for afford that I've seen before include "Provide; manage to pay for" , "Be able to spare the money" , "Run to" , "Be able to buy" , "Have enough money to pay for" .)