Decide not to join old heartless Irishman going on holiday (3,3)
I believe the answer is:
opt out
'decide not to join' is the definition.
(I've seen this in another clue)
'old heartless irishman going on holiday' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'heartless' means to remove the middle letters.
'irishman' becomes 'Pat' (Patrick is a common Irish name).
'going on holiday' becomes 'out' (I can't explain this - if you can you should give a lot more credence to this answer).
'pat' with its centre removed is 'pt'.
'o'+'pt'+'out'='OPT-OUT'
Can you help me to learn more?
(Other definitions for opt out that I've seen before include "Choose not to do something" , "'Choose not to take part (3,3)'" , "Decide not to participate" , "Decline to participate" , "Choose to be excluded" .)