Group occasionally runs by a lake in the country (5)
I believe the answer is:
rural
'in the country' is the definition.
(I've seen this before)
'group occasionally runs by a lake' is the wordplay.
'occasionally' means one should take alternating letters.
'runs' becomes 'r' (cricket abbreviation**).
'by' says to put letters next to each other.
'lake' becomes 'l'.
The alternating letters of 'group' are 'ru'.
'ru'+'r'+'a'+'l'='RURAL'
(Other definitions for rural that I've seen before include "outside the city?" , "Relating to country life" , "Characteristic of country life" , "Countryfied" , "covering the country" .)