A thousand for every initial symbol and sign (9)
I believe the answer is:
ampersand
'sign' is the definition.
(ampersand is a kind of sign)
'a thousand for every initial symbol and' is the wordplay.
'thousand' becomes 'm' (synonyms).
'for every' becomes 'per' (I've seen this before).
'initial' indicates taking the first letters.
The first letter of 'symbol' is 's'.
'a'+'m'+'per'+'s'+'and'='AMPERSAND'
(Other definitions for ampersand that I've seen before include "Union character" , "Symbol for 'and'" , "Pens drama using &" , "Plus sign" , "It connects" .)