Not being fully up to the task, had a rest (3)
I believe the answer is:
sat
'had a rest' is the definition.
(sat down)
'not being fully up to the task' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
The remaining letters 'sa' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sat that I've seen before include "Was in session" , "Rested" , "twenty-four hours" , "Took a chair" , "Relaxed" .)