Carp in Cairo's water closing in on horse (6)
I believe the answer is:
niggle
'carp' is the definition.
(I've seen this in another clue)
'cairo's water closing in on horse' is the wordplay.
I cannot quite see how this works, but
'horse' could be 'gg' (geegee or GG is a childish term for a horse) and 'gg' is found in the answer.
'water' could be 'nile' (Nile is an example) and 'nile' is present in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for niggle that I've seen before include "Minor criticism - slight worry" , "Get" , "Trivial problem, fuss" , "Petty annoyance" , "A slight bother" .)