Just Mad Hatter's act (2,1,7)
I believe the answer is:
at a stretch
'just' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'mad hatter's act' is the wordplay.
'mad' indicates an anagram.
'hatters'+'act'='hattersact'
'hattersact' anagrammed gives 'AT A STRETCH'.
Can you help me to learn more?
(Other definitions for at a stretch that I've seen before include "in one continuous period" , "With much effort" , "Running, possibly" , "With difficulty; without interruption" .)