What pairs of clues do, first/second, third/fourth ... (5)
I believe the answer is:
rhyme
'what pairs of clues do' is the definition.
The definition suggests a singular noun which matches the answer.
'firstsecond thirdfourth' is the wordplay.
I cannot really understand how this works, but
'firstsecond' could be 'e' (1st letter of 'econd') and 'e' is located in the answer.
'thirdfourth' could be 'r' (4th letter of 'thir') and 'r' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rhyme that I've seen before include "Correspondence in the sounds of two or more lines" , "Short piece of verse" , "Have identical terminal sounds" , "Correspondence in the sounds of two or more words" , "Bit of poetry" .)