Captain treated initially for disease (6)
I believe the answer is:
blight
'for disease' is the definition.
(blight is a kind of disease)
'captain treated initially' is the wordplay.
'captain' becomes 'bligh' (I've seen this before).
'treated initially' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bligh'+'t'='BLIGHT'
Can you help me to learn more?
(Other definitions for blight that I've seen before include "Fungal plant disease" , "Disease or fungus that withers plants, say potatoes" , "Disease of plants etc" , "Affliction" , "Spoil - plant disease" .)