It's no good if you apply a smaller quantity (7)
I believe the answer is:
useless
'it's no good' is the definition.
(I've seen this before)
'apply a smaller quantity' is the wordplay.
'apply' becomes 'use' (synonyms).
'a smaller quantity' becomes 'less' (I've seen this before).
'use'+'less'='USELESS'
'if you' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for useless that I've seen before include "Unserviceable" , "Lacking in purpose" , "Without any worthwhile application" , "Unemployable" , "Ineffective" .)