Fungus not severe; we came back (6)
I believe the answer is:
mildew
'fungus' is the definition.
(mildew is a kind of fungus)
'not severe we came back' is the wordplay.
'not severe' becomes 'mild' (I've seen this before).
'came back' is a reversal indicator.
'we' written backwards gives 'ew'.
'mild'+'ew'='MILDEW'
(Other definitions for mildew that I've seen before include "Fungal plant disease" , "attack of disease" , "Fungal disease in plants" , "Fungi growing on plants" , "Fungus that produces mould on damp material" .)