Force union leader to wear frock (6)
I believe the answer is:
duress
'force' is the definition.
(duress is a kind of force)
'union leader to wear frock' is the wordplay.
'leader' indicates taking the first letters.
'to wear' means one lot of letters goes inside another (to wear something is to be covered up by it).
'frock' becomes 'dress' (synonyms).
The initial letter of 'union' is 'u'.
'u' going into 'dress' is 'DURESS'.
(Other definitions for duress that I've seen before include "Illegal coercion" , "Constraint, force" , "Constraint, coercion" , "Threat of coercion" , "Force, coercion" .)