Tasteless cool sauce is sent back (7)
I believe the answer is:
insipid
'tasteless' is the definition.
(similar in meaning)
'cool sauce is sent back' is the wordplay.
'cool' becomes 'in' (I've seen this in other clues).
'sauce' becomes 'dip' (I've seen this before).
'sent back' is a reversal indicator.
'dip'+'is'='dipis'
'dipis' back-to-front is 'sipid'.
'in'+'sipid'='INSIPID'
(Other definitions for insipid that I've seen before include "Lacking 10" , "Bland, flavourless" , "Lacking taste or flavour" , "Lacking flavour or banal" , "flat" .)