Austere day as little jerk (8)
I believe the answer is:
monastic
'austere' is the definition.
(I've seen this in another clue)
'day as little jerk' is the wordplay.
'day' becomes 'mon' (short for Monday).
'little jerk' becomes 'tic' (jerking movement by the body).
'mon'+'as'+'tic'='MONASTIC'
(Other definitions for monastic that I've seen before include "Austere" , "Of sequestered life under religious vows" , "Brother's" , "Relating to life in a cloister" , "Isolated" .)