The blue salad ingredient is mushroom (9)
I believe the answer is:
skyrocket
'is mushroom' is the definition.
(I've seen this before)
'the blue salad ingredient' is the wordplay.
'the blue' becomes 'sky' (sky blue is a shade of blue).
'salad ingredient' becomes 'rocket' (I've seen this before).
'sky'+'rocket'='SKYROCKET'
(Other definitions for skyrocket that I've seen before include "Mushroom" , "Rapidly appreciate" , "seen to shoot up" , "Soar" , "Shoot up abruptly" .)