The pressure to be uniform in attire (6)
I believe the answer is:
Here is my best explanation:
'the pressure' is the definition.
'uniform in attire' is the subsidiary indication.
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'attire' becomes 'dress' (dressing is a kind of attiring).
'u' inserted inside 'dress' is 'duress'.
'to be' is the link.
(Other definitions for duress that I've seen before include "Coercion, pressure", "Constraint, coercion", "Force, coercion", "Restraint", "Use of force or threats, often vile", "Compulsion by use of force", "Threat of coercion", "Pressure (involving threats)", "Constraint, force" and "Constraint, Compulsion".)