Allergy symptom spreading threatens to devour learner (10)
I believe the answer is:
nettlerash
'allergy' is the definition.
(I have seen 'Painful allergy' mean 'nettlerash' so perhaps 'allergy' could also mean 'nettlerash')
'spreading threatens to devour learner' is the wordplay.
'spreading' is an insertion indicator (inserted letters spread others open).
'to devour' indicates an anagram (I've seen 'devoured' mean this).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'threatens' with letters rearranged gives 'netterash'.
'netterash' enclosing 'l' is 'NETTLERASH'.
'symptom' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for nettlerash that I've seen before include "irritating complaint" , "Painful allergy" , "hives" .)