Zoo's first uniform in time turned blue (5)
I believe the answer is:
azure
'blue' is the definition.
(azure is a kind of blue)
'zoo's first uniform in time turned' is the wordplay.
'zoo's first' becomes 'z' (1st letter of 'zoo').
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' is an insertion indicator.
'time' becomes 'era' (era is a kind of time).
'turned' is a reversal indicator.
'era' backwards is 'are'.
'z'+'u'='zu'
'zu' put inside 'are' is 'AZURE'.
(Other definitions for azure that I've seen before include "Delicate blue" , "Heraldic blue" , "Sky-coloured" , "Deep sky-blue colour" , "Sort of blue" .)