Massage Sally’s back, which is a deep red (4)
I believe the answer is:
ruby
'a deep red' is the definition.
(synonyms)
'massage sally's back' is the wordplay.
'massage' becomes 'rub' (rub can mean to massage).
'back' suggests the final letters (the letter at the back).
The final letter of 'sally' is 'y'.
'rub'+'y'='RUBY'
'which is' acts as a link.
(Other definitions for ruby that I've seen before include "Girl's name - colour" , "Stony girl" , "Red precious stone" , "Vivid red" , "Fortieth anniversary stone" .)