Target of left-leaning clubs, periodically (5)
I believe the answer is:
focus
'target' is the definition.
(I've seen this before)
'of left-leaning clubs periodically' is the wordplay.
'left-leaning' shows that the letters should be reversed in order (letters written right to left).
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternate letters of 'clubs' are 'cus'.
'of' in reverse letter order is 'fo'.
'fo'+'cus'='FOCUS'
(Other definitions for focus that I've seen before include "Target" , "Main topic of attention" , "Bull's eye" , "Central point" , "Hub" .)