Left student absorbed by reason mainly for a thickening of tissue (6)
I believe the answer is:
callus
'a thickening of tissue' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'left student absorbed by reason mainly' is the wordplay.
'left' becomes 'l' (common abbreviation).
'student' becomes 'l' (as in L-plates for learner drivers).
'absorbed by' means one lot of letters goes inside another.
'reason' becomes 'cause' (I've seen this in another clue).
'mainly' means to remove the last letter.
'l'+'l'='ll'
'cause' with its final letter taken off is 'caus'.
'll' going inside 'caus' is 'CALLUS'.
'for' is the link.
Can you help me to learn more?
(Other definitions for callus that I've seen before include "Area of hard skin" , "This thick-skinned" , "Thickened skin patch" , "Corn" , "Skin problem" .)