Working at mode - quantity is bespoke (4-2-7)
I believe the answer is:
made-to-measure
'is bespoke' is the definition.
(I've seen this before)
'working at mode quantity' is the wordplay.
'working' indicates an anagram.
'quantity' becomes 'measure' (synonyms).
'at'+'mode'='atmode'
'atmode' is an anagram of 'madeto'.
'madeto'+'measure'='MADE-TO-MEASURE'
(Other definitions for made-to-measure that I've seen before include "Specially tailored" , "Not off the peg" , "Custom-built; bespoke" , "Not off-the-peg" .)