Growing reason why I’m in sore distress (5)
I believe the answer is:
osier
'growing reason why' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'i'm in sore distress' is the wordplay.
'i'm' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'distress' is an anagram indicator.
'sore' with letters rearranged gives 'oser'.
'i' placed inside 'oser' is 'OSIER'.
Can you help me to learn more?
(Other definitions for osier that I've seen before include "Twig used in basket-making" , "branch" , "Flexible twig of a willow tree" , "Bit of tree" , "one used to get weaving?" .)