Uniform sporting tin star (3)
I believe the answer is:
sun
'star' is the definition.
(the Sun is a star)
'uniform sporting tin' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'sporting' is an insertion indicator (inserted letters wear or 'sport' others).
'tin' becomes 'Sn' (Sn is the chemical symbol for tin).
'u' placed inside 'sn' is 'SUN'.
(Other definitions for sun that I've seen before include "Rag" , "Star that provides light and heat for a solar system" , "Dawn riser" , "Star round which we orbit" , "Star; paper" .)