One stimulant in a hundred, I see (5)
I believe the answer is:
tonic
'one stimulant' is the definition.
(thesaurus)
'a hundred i see' is the wordplay.
'a hundred' becomes 'ton' (I've seen this before).
'see' becomes 'c' (the word for the letter, according to Chambers).
'ton'+'i'+'c'='TONIC'
'in' is the link.
(Other definitions for tonic that I've seen before include "A pick-me-up drink, nice with gin" , "Producing tension of muscles" , "medicinal preparation" , "shot in the arm" , "Carbonated mixer" .)