Turn fifty, oddly lacking detachment (4)
I believe the answer is:
unit
'detachment' is the definition.
(I've seen this before)
'turn fifty oddly lacking' is the wordplay.
'oddly lacking' means one should take alternating letters.
'turn'+'fifty'='turnfifty'
The alternate letters of 'turnfifty' are 'UNIT'.
(Other definitions for unit that I've seen before include "Single person - group of people" , "Single item; one" , "Some fighting men" , "Constituent" , "Operational team" .)