You shouldn't have holiday sex with wretched Keir to please yourself (4,2,2,5,2)
I believe the answer is:
take it or leave it
'please yourself' is the definition.
(I've seen this before)
'you shouldn't have holiday sex with wretched keir to' is the wordplay.
'you shouldn't have' becomes 'ta' (both can mean 'thank you').
'holiday' becomes 'leave' (I've seen this before).
'sex' becomes 'it' (a common euphemism).
'with' means one lot of letters go next to another.
'wretched' is an anagram indicator.
'keir'+'to'='keirto'
'keirto' anagrammed gives 'keitor'.
'leave'+'it'='leaveit'
'leaveit' after 'keitor' is 'keitorleaveit'.
'ta'+'keitorleaveit'='TAKE IT OR LEAVE IT'
(Other definitions for take it or leave it that I've seen before include "Please yourself" , "Two options for the gin drinker" , "could be the thief's dilemma" , "no more concessions" , "Clear choice for one tempted to thieve" .)