Support cutting first section of old book - it's understandable (7)
I believe the answer is:
legible
'support cutting' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?
'first section of old book it's understandable' is the wordplay.
I cannot quite see how this works, but
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is located in the answer.
'section' could be 'leg' (as in a section of competition) and 'leg' is located in the answer.
'book' could be 'b' (abbreviation for book) and 'b' is present in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found in the answer.
A single letter 'l' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for legible that I've seen before include "Easy to read" , "possible to make out" , "Readable" , "Easily read" , "Able to be read" .)