Supply crack on base (5)
I believe the answer is:
equip
'supply' is the definition.
(equipping is a kind of supplying)
'crack on base' is the wordplay.
'crack' becomes 'quip' (both can mean a joke).
'on' means one lot of letters go next to another.
'base' becomes 'e' (Euler's number - base of natural logarithms).
'quip' put after 'e' is 'EQUIP'.
(Other definitions for equip that I've seen before include "Fit out, supply" , "Give kit to" , "Provide with requirements" , "Provide with stock" , "Outfit" .)