It irritates them when horse is immersed in river by son (7)
I believe the answer is:
niggles
'it irritates them' is the definition.
The definition suggests a singular noun but the answer is not.
'horse is immersed in river by son' is the wordplay.
'horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'is immersed in' indicates putting letters inside.
'river' becomes 'nile' (Nile River is an example).
'by' means one lot of letters go next to another.
'son' becomes 's' (genealogical abbreviation for son).
'gg' going within 'nile' is 'niggle'.
'niggle'+'s'='NIGGLES'
'when' is the link.
Can you help me to learn more?
(Other definitions for niggles that I've seen before include "Slight but persistent anxieties" , "Small worries" , "minor injuries" , "Petty but persistent annoyances" .)