Monk, say, when jerk joins church (7)
I believe the answer is:
ascetic
'monk' is the definition.
(I've seen this in another clue)
'when jerk joins church' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'jerk' becomes 'tic' (jerking movement by the body).
'joins' says to put letters next to each other.
'church' becomes 'ce' (abbreviation for Church of England).
'tic' after 'ce' is 'cetic'.
'as'+'cetic'='ASCETIC'
'say' acts as a link.
(Other definitions for ascetic that I've seen before include "Self-denying, leading severely simple life" , "Leading a severely simple life without worldly pleasures" , "One practising self-denial as a discipline" , "Frugal" , "One living strict life" .)