Mother on river with second group finding monkey (8)
I believe the answer is:
marmoset
'monkey' is the definition.
(marmoset is a kind of monkey)
'mother on river with second group' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'river' becomes 'r' (abbreviation for river).
'with second' becomes 'mo' (both are short periods of time. I am not sure about the 'with' bit.).
'group' becomes 'set' (both can mean a collection).
'ma'+'r'+'mo'+'set'='MARMOSET'
'finding' acts as a link.
Can you help me to learn more?
(Other definitions for marmoset that I've seen before include "Small tropical American monkey" , "rather small creature" , "Somme rat? (anag)" , "Small monkey - Rome's mat (anag)" , "Primate" .)