For many, I see, it's a kind of acid (6)
I believe the answer is:
formic
'a kind of acid' is the definition.
'for many i see' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'see' becomes 'C' (the word for the letter, according to Chambers).
'for'+'m'+'i'+'c'='FORMIC'
'it's' acts as a link.
(Other definitions for formic that I've seen before include "Colourless acid emitted by ants and nettles" , "Type of acid made by some ants" , "Acid contained in nettles" .)