Letter from Rolling Stone clutched by woman now and then (5)
I believe the answer is:
omega
'letter' is the definition.
(I know that omega is a type of letter)
'rolling stone clutched by woman now and then' is the wordplay.
'rolling' says the letters should be written in reverse.
'stone' becomes 'gem' (gemstone).
'clutched by' is an insertion indicator.
'now and then' indicates alternate letters (letters taken on a regular basis, now and then).
The alternating letters of 'woman' are 'oa'.
'gem' in reverse letter order is 'meg'.
'meg' put into 'oa' is 'OMEGA'.
'from' acts as a link.
(Other definitions for omega that I've seen before include "symbol of resistance?" , "Final outcome" , "The last in a series of letters" , "Ultimate letter" , "Art of story-telling by body movement" .)