Monitored group of swimmers infested by parasites (7)
I believe the answer is:
policed
'monitored' is the definition.
(I've seen this in another clue)
'group of swimmers infested by parasites' is the wordplay.
'group of swimmers' becomes 'pod' (I am not sure about this - if you are sure you should believe this answer much more).
'infested by' is an insertion indicator.
'parasites' becomes 'lice' (lice are parasitic insects).
'pod' placed around 'lice' is 'POLICED'.
Can you help me to learn more?
(Other definitions for policed that I've seen before include "Patrolled" , "Used law-enforcement methods" , "maintained order" .)