Prompt on day off (5)
I believe the answer is:
ready
'prompt' is the definition.
('ready' can be similar in meaning to 'prompt')
'on day off' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'off' indicates an anagram.
'day' anagrammed gives 'ady'.
're'+'ady'='READY'
(Other definitions for ready that I've seen before include "game?" , "Willing (to do something); cash" , "Willing (to do something), prepare" , "Prepared, willing" , "rough companion" .)