It's unnecessary to require a smaller amount (8)
I believe the answer is:
needless
'it's unnecessary' is the definition.
(similar in meaning)
'require a smaller amount' is the wordplay.
'require' becomes 'need' (synonyms).
'a smaller amount' becomes 'less' (I've seen this before).
'need'+'less'='NEEDLESS'
'to' is the link.
(Other definitions for needless that I've seen before include "Superfluous" , "Unneccessary and unwarranted" , "uncalled for" , "Unnecessary, avoidable" , "- - to say" .)