Set out on the wrong day in uniform (6)
I believe the answer is:
steady
'uniform' is the definition.
(I've seen this before)
'set out on the wrong day' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'on' is a charade indicator (letters next to each other).
'the wrong' indicates anagramming the letters.
'set' is an anagram of 'ste'.
'day' is an anagram of 'ady'.
'ste'+'ady'='STEADY'
'in' is the link.
(Other definitions for steady that I've seen before include "Firmly supported or balanced" , "not easily shaken" , "Poised" , "Not liable to fluctuate" , "Ready . . . . . . go" .)