Now and again wants post that's up for grabs (2,5)
I believe the answer is:
at stake
'up for grabs' is the definition.
(something at stake is winnable or up for grabs)
'now and again wants post' is the wordplay.
'now and again' indicates alternate letters (letters taken on a regular basis).
'post' becomes 'stake' (synonyms).
The alternate letters of 'wants' are 'at'.
'at'+'stake'='AT STAKE'
'that's' is the link.
(Other definitions for at stake that I've seen before include "Being risked, like money with bookie" , "Being risked or bet on" , "Being risked, depending on outcome of event" , "To be won or lost" , "In danger; at issue" .)