Goodbye to a new idea for a uniform (5)
I believe the answer is:
adieu
'goodbye' is the definition.
('adieu' can be a synonym of 'goodbye')
'a new idea for a uniform' is the wordplay.
'a new' is an anagram indicator.
'for' says to put letters next to each other.
'a uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'idea' with letters rearranged gives 'adie'.
'adie'+'u'='ADIEU'
'to' is the link.
(Other definitions for adieu that I've seen before include "Farewell, adios" , "farewell message" , "Farewell (French)" , "Goodbye, mademoiselle" , "A valediction" .)