Offer support with constable oddly off (8)
I believe the answer is:
proposal
'offer' is the definition.
(proposal is a kind of offer)
'support with constable oddly off' is the wordplay.
'support' becomes 'prop' (prop is a kind of support).
'with' means one lot of letters go next to another.
'oddly off' means one should take alternating letters (letters in odd-numbered positions removed).
The alternate letters of 'constable' are 'osal'.
'prop'+'osal'='PROPOSAL'
(Other definitions for proposal that I've seen before include "Offer - suggestion" , "motion" , "A suggestion, perhaps of marriage" , "Offer, of marriage say" , "Offer (to get spliced?)" .)