Not intend shag? That's that ruined! (3-5,5)
I believe the answer is:
one-night stand
'not intend shag? that's that ruined' is the definition.
I can't judge whether this defines the answer.
'not intend shag? that's that ruined' is the wordplay.
'that's that ruined' indicates anagramming the letters.
'not'+'intend'+'shag'='notintendshag'
'notintendshag' anagrammed gives 'ONE-NIGHT STAND'.
(Other definitions for one-night stand that I've seen before include "Brief amour" , "Not a lasting affair" , "little time for love" , "Performance confined to a single evening" , "Brief personal encounter -- single performance" .)