Plant of the mustard family shock alien? (6)
I believe the answer is:
rocket
'plant of the mustard' is the definition.
'rocket' can be an answer for 'plant' (rocket is a kind of plant). I'm not sure about the 'of the mustard' bit.
'family shock alien?' is the wordplay.
'family shock' becomes 'rock' (I can't justify this - if you can you should give a lot more credence to this answer).
'alien?' becomes 'et' (extra terrestrial).
'rock'+'et'='ROCKET'
Can you help me to learn more?
(Other definitions for rocket that I've seen before include "Projectile salad ingredient" , "Salad leaves - move at great speed" , "Salad plant; strong reprimand" , "that should take off?" , "Self-propelled missile and a plant" .)