Monday bad for energetic person (6)
I believe the answer is:
dynamo
'energetic person' is the definition.
(I've seen this in another clue)
'monday bad' is the wordplay.
'bad' indicates an anagram.
'monday' is an anagram of 'DYNAMO'.
'for' is the link.
(Other definitions for dynamo that I've seen before include "Energetic person (colloq.)" , "Monday (anag)" , "Generator (sometimes 'human'?)" , "activist?" , "energetic individual" .)