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' put after 'ce' is 'cetic'.
'as'+'cetic'='ASCETIC'
'say' is the 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" .)