Catchy tune's beginning to jar and grate (6)
I believe the answer is:
jingle
'catchy tune's' is the definition.
(I've seen this before)
'beginning to jar and grate' is the wordplay.
'beginning to' indicates taking the first letters.
'and' says to put letters next to each other.
'grate' becomes 'ingle' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letter of 'jar' is 'j'.
'j'+'ingle'='JINGLE'
Can you help me to learn more?
(Other definitions for jingle that I've seen before include "Catchy tune in advert" , "Catchy advertising tune - Pickwick Papers character" , "Memorable slogan/tune" , "Marketing slogan" , "Sound of shaken coins" .)