A small amount mostly leads to vomit? Just conceivably (2,1,7)
I believe the answer is:
at a stretch
'just conceivably' is the definition.
'a small amount mostly leads to' is the wordplay.
'small amount' becomes 'taste'.
'mostly' means to remove the last letter.
'leads to' becomes 'retch' (I can't justify this - if you can you should give a lot more credence to this answer).
'taste' with its last letter taken off is 'tast'.
'a'+'tast'+'retch'='AT A STRETCH'
'vomit?' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
(Other definitions for at a stretch that I've seen before include "in one continuous period" , "With difficulty; without interruption" , "With much effort" , "Running, possibly" .)