Drive outrigger periodically (4)
I believe the answer is:
urge
'drive' is the definition.
(I've seen this before)
'outrigger periodically' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternate letters of 'outrigger' are 'URGE'.
(Other definitions for urge that I've seen before include "Encourage to proceed, drive onward" , "Advise strongly to press forward" , "'Spur on, encourage (4)'" , "Persuade - proclivity" , "Encourage or entreat earnestly" .)