Not eating al desko, having lost the plot (3,2,5)
I believe the answer is:
out to lunch
'plot' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'not eating al desko having lost the' is the wordplay.
I cannot really understand how this works, but
'not' could be 'un' (prefix meaning 'not') and 'un' is present in the answer.
'lost' could be 'l' (abbreviation in sporting league tables) and 'l' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for out to lunch that I've seen before include "unhinged?" , "being served in restaurant perhaps" , "stupid" , "Getting fed up in office hours" , "Crackers" .)