River contains more than one grand carp (6)
I believe the answer is:
niggle
'carp' is the definition.
(I've seen this in another clue)
'river contains more than one grand' is the wordplay.
'river' becomes 'nile' (Nile River is an example).
'contains' means one lot of letters goes inside another.
'more than' becomes 'g' (I can't justify this - if you can you should give a lot more credence to this answer).
'one grand' becomes 'g' (G is short for grand as in a thousand).
'g'+'g'='gg'
'nile' going around 'gg' is 'NIGGLE'.
Can you help me to learn more?
(Other definitions for niggle that I've seen before include "Find fault in a petty way, fuss over details" , "Petty annoyance" , "Trivial criticism or annoyance" , "Annoy slightly" , "Objection" .)