Sing rhythm and blues principally visiting most of country (6)
I believe the answer is:
warble
'sing rhythm' is the definition.
'warble' can be an answer for 'sing' (warbling is a kind of singing). I am not certain of the 'rhythm' bit.
'blues principally visiting most of country' is the wordplay.
'blues principally' becomes 'rb' (I can't explain this - if you can you should believe this answer much more).
'visiting' is an insertion indicator.
'most of' means to remove the last letter.
'country' becomes 'wales' (country of the UK).
'wales' with its last letter taken away is 'wale'.
'rb' inserted inside 'wale' is 'WARBLE'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for warble that I've seen before include "Sing with gentle trilling note like some birds" , "Twitter" , "Sing - type of fly" , "Sing like a thrush" , "Sing with trilling note like a bird" .)