Exercise reduced in vain (7)
I believe the answer is:
useless
'vain' is the definition.
(I've seen this before)
'exercise reduced' is the wordplay.
'exercise' becomes 'use' (synonyms).
'reduced' becomes 'less'.
'use'+'less'='USELESS'
'in' acts as a link.
(Other definitions for useless that I've seen before include "Poor" , "Serving no practical purpose" , "Ineffectual" , "Of no practical or worthwhile application" , "Incompetent" .)