Tasteless sauce mostly isn’t one for picking up (7)
I believe the answer is:
insipid
'tasteless' is the definition.
('insipid' can be a synonym of 'tasteless')
'sauce mostly isn't one for picking up' is the wordplay.
'sauce' becomes 'dip' (I've seen this before).
'mostly' means to remove the last letter.
'one' becomes 'i' (Roman numeral**).
'for picking up' is a reversal indicator.
'isnt' with its last letter taken off is 'isn'.
'dip'+'isn'+'i'='dipisni'
'dipisni' reversed gives 'INSIPID'.
(Other definitions for insipid that I've seen before include "Boring and lacking liveliness" , "flat" , "Lacking flavour or banal" , "Wishy-washy" , "Lacking 10" .)