Working shifts you tend to lack energy (2,4)
I believe the answer is:
on duty
'working' is the definition.
(thesaurus)
'shifts you tend to lack energy' is the wordplay.
'shifts' is an anagram indicator.
'to lack' indicates named letters should be taken away.
'energy' becomes 'e' (physics symbol as in E=mc2).
'you'+'tend'='youtend'
'youtend' with 'e' removed is 'youtnd'.
'youtnd' is an anagram of 'ON-DUTY'.
(Other definitions for on duty that I've seen before include "(Of soldier) working" , "What working nurses are" , "Working at a particular time" , "As working policemen are" , "Engaged in one's regular work" .)