Groups of salmon right at mouth of stream (7)

'groups' is the definition.
The answer is an example.

'salmon right at mouth of stream' is the wordplay.
'salmon' becomes 'coho' (type of salmon).
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'at' is a charade indicator (letters next to each other).
'mouth of' indicates taking the first letters (mouth can mean something's origin).
The initial letter of 'stream' is 's'.
'coho'+'rt'+'s'='COHORTS'

'of' acts as a link.