Goitre — lack of Iodine may lead to this disease (5)
I believe the answer is:
ergot
'goitre lack of iodine may lead to this disease' is the definition.
'ergot' can be an answer for 'disease' (ergot poisoning). I'm not certain of the rest of the definition.
'iodine may lead to this disease' is the wordplay.
'iodine may' becomes 'erg' (I can't explain this - if you can you should believe this answer much more).
'lead' says to put letters next to each other.
'this disease' indicates anagramming the letters (I've seen 'diseased' mean this (I've seen this in other clues)).
'to' anagrammed gives 'ot'.
'erg'+'ot'='ERGOT'
Can you help me to learn more?
(Other definitions for ergot that I've seen before include "Parasitical fungus" , "One across disease of rye" , "Disease of rye and other cereals" , "Fungal disease of grain" , "Fungal disease of cereals" .)