A good person retaining muscle for particular part (6)
I believe the answer is:
aspect
'particular part' is the definition.
(I've seen this before)
'a good person retaining muscle' is the wordplay.
'good person' becomes 'st' (saint).
'retaining' is an insertion indicator (inserted letters are held or retained).
'muscle' becomes 'pec' (pectoral muscle).
'st' enclosing 'pec' is 'spect'.
'a'+'spect'='ASPECT'
'for' acts as a link.
(Other definitions for aspect that I've seen before include "Distant view of a wide area" , "Visual effect" , "Vista, panorama" , "'Appearance, facet (6)'" , "'Scene, view, vista (6)'" .)