Read a letter when you’re able (4)
I believe the answer is:
scan
'read a letter' is the definition.
'scan' can be an answer for 'read' (scanning is a kind of reading). I'm not certain of the 'a letter' bit.
'you're able' is the wordplay.
'you're' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'able' becomes 'can'.
's'+'can'='SCAN'
'when' is the link.
Can you help me to learn more?
(Other definitions for scan that I've seen before include "Survey - check-up" , "Medical procedure" , "Examine hastily" , "Look over (in order to copy?)" , "Examine intensely -- examine superficially" .)