Margaret's back with rough notes for Beryl, say
I believe the answer is:
gemstone
'beryl say' is the definition.
(beryl is a kind of gemstone)
'margaret's back with rough notes' is the wordplay.
'margaret' becomes 'meg' (short for Margaret).
'back' is a reversal indicator.
'with' means one lot of letters go next to another.
'rough' is an anagram indicator (a rough or broken ordering of the letters).
'meg' in reverse letter order is 'gem'.
'notes' with letters rearranged gives 'stone'.
'gem'+'stone'='GEMSTONE'
'for' is the link.
(Other definitions for gemstone that I've seen before include "Sparkler" , "Eg, sapphire" , "Emerald or ruby" , "Physical power" , "Something precious" .)