PM has about a thousand in support (7)
I believe the answer is:
premier
'pm' is the definition.
(synonyms)
'about a thousand in support' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'a thousand' becomes 'M' (Roman numeral for a thousand).
'in' means one lot of letters goes inside another.
'support' becomes 'pier' (pier is a kind of support).
're'+'m'='rem'
'rem' placed into 'pier' is 'PREMIER'.
'has' acts as a link.
(Other definitions for premier that I've seen before include "Which is having precedence" , "PM" , "First in importance" , "First (minister)" , "Chief minister" .)