Supply on-line gag? (5)
I believe the answer is:
equip
'supply' is the definition.
(I know that equip is a more specific form of the action supply)
'on-line gag?' is the wordplay.
'on-line' becomes 'e' (abbreviation for electronic, e.g. in ecommerce).
'gag?' becomes 'quip' ('quip' can be a synonym of 'gag').
'e'+'quip'='EQUIP'
(Other definitions for equip that I've seen before include "Supply with what is needed" , "Give kit to" , "Pique (anag.)" , "Provide stuff" , "Fit out, supply" .)