Compulsion to line up carrying uniform (6)
I believe the answer is:
duress
'compulsion' is the definition.
(I've seen this in another clue)
'line up carrying uniform' is the wordplay.
'line up' becomes 'dress' (synonyms).
'carrying' indicates putting letters inside.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'dress' placed around 'u' is 'DURESS'.
'to' is the link.
(Other definitions for duress that I've seen before include "Excessive pressure" , "Hardship" , "Force, coercion" , "Compulsion by use of force" , "Use of force or threats, often vile" .)