Observe then follow monitoring group (8)
I believe the answer is:
watchdog
'monitoring group' is the definition.
(I've seen this before)
'observe then follow' is the wordplay.
'observe' becomes 'watch' (synonyms).
'then' says to put letters next to each other.
'follow' becomes 'dog' (dog can mean to follow or torment).
'watch'+'dog'='WATCHDOG'
(Other definitions for watchdog that I've seen before include "Guardian of people's rights, of canine origin" , "Animal guard" , "Ombudsman" , "Monitoring organisation" , "TV show" .)