Pungent family of bulbs, with fifty-one preserved in alum (6)
I believe the answer is:
allium
'pungent family of bulbs' is the definition.
I can't tell whether this defines the answer.
'fifty-one preserved in alum' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'one preserved' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'l'+'i'='li'
'li' placed within 'alum' is 'ALLIUM'.
'with' is the link.
Can you help me to learn more?
(Other definitions for allium that I've seen before include "Plant of onion family" , "Plant of onion genus" , "Bulbous plant" , "Onion/garlic genus" .)