The coasts of Cornwall get attention - it's understandable (5)
I believe the answer is:
clear
'understandable' is the definition.
('clear' can be similar in meaning to 'understandable')
'the coasts of cornwall get attention' is the wordplay.
'the coasts of cornwall' becomes 'cl' (I can't explain this - if you can you should give a lot more credence to this answer).
'get' says to put letters next to each other.
'attention' becomes 'ear' (ear is a kind of attention).
'cl'+'ear'='CLEAR'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for clear that I've seen before include "Disengaged" , "Easy to see or understand" , "With no jams" , "Plainly" , "Exculpate" .)