Exercise system of irrational evening shifts? (7)
I believe the answer is:
pilates
'exercise system' is the definition.
(pilates is an exercise method)
'irrational evening shifts?' is the wordplay.
'irrational' becomes 'pi' (an irrational number).
'evening shifts?' becomes 'lates'.
'pi'+'lates'='PILATES'
'of' acts as a link.
(Other definitions for pilates that I've seen before include "Exercise system" , "Ali's pet (anag) - exercise regime" , "regime for improved fitness" , "attributed to Biblical procurator?" , "Modern exercise" .)