Start to fray and become tiresome (4,4)
I believe the answer is:
wear thin
'become tiresome' is the definition.
(I've seen this in another clue)
'start to fray and' is the wordplay.
'start' becomes 'thi' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' says to put letters next to each other (I've seen this in other clues).
'fray' becomes 'wear' (I've seen this in another clue).
'and' becomes 'n' (common abbreviation for 'and').
'thi' put after 'wear' is 'wearthi'.
'wearthi'+'n'='WEAR THIN'
Can you help me to learn more?
(Other definitions for wear thin that I've seen before include "Lose effect from overuse" , "Gradually dwindle" , "Become less convincing through overuse" , "Diminish - become less tolerable" , "Gradually become less persuasive" .)