Non-drinker needing support over summer (11)
I believe the answer is:
teetotaller
'non-drinker' is the definition.
(I've seen this before)
'support over summer' is the wordplay.
'support' becomes 'tee' (tee is a kind of support).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'summer' becomes 'totaller' (someone summing or totalling things up).
'tee'+'totaller'='TEETOTALLER'
'needing' is the link.
(Other definitions for teetotaller that I've seen before include "Abstainer from drink" , "Abstainer from alcohol" , "Rechabite" , "One dry" , "Non-drinker" .)