Muscle pain in this setter's bad legs I saw periodically (7)
I believe the answer is:
myalgia
'muscle pain' is the definition.
(I've seen this before)
'this setter's bad legs i saw periodically' is the wordplay.
'this setter's' becomes 'my' (I've seen this in other clues**).
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'bad'+'legs'+'i'+'saw'='badlegsisaw'
The alternating letters of 'badlegsisaw' are 'algia'.
'my'+'algia'='MYALGIA'
'in' acts as a link.
(Other definitions for myalgia that I've seen before include "painful muscle condition" , "Muscle pain" , "Muscular pain" , "sort of pain!" .)