Support the wrong forecaster (7)
I believe the answer is:
prophet
'forecaster' is the definition.
(I've seen this before)
'support the wrong' is the wordplay.
'support' becomes 'prop' (prop up can mean to support).
'wrong' indicates an anagram.
'the' is an anagram of 'het'.
'prop'+'het'='PROPHET'
(Other definitions for prophet that I've seen before include "Say, Cassandra" , "Augur" , "Soothsayer" , "Moses, for example" , "Oracle" .)