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' is a reversal indicator (letters written right to left).
'periodically' indicates alternate letters (letters must be selected 'every so often').
The alternate letters of 'clubs' are 'cus'.
'of' back-to-front 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" .)