Cask beer picked up for military detachment (5)
I believe the answer is:
draft
'military detachment' is the definition.
'cask beer picked up' is the wordplay.
'cask beer' becomes 'draught'.
'picked up' indicates a 'sounds like' (homophone) clue.
'draught' is a homophone of 'DRAFT'.
'for' is the link.
(Other definitions for draft that I've seen before include "Early version of text" , "Written plan" , "Provisional document" , "Rough version" , "conscript" .)