Strangely sullen about uniform with small crescents (7)

'small crescents' is the definition.

'strangely sullen about uniform' is the wordplay.
'strangely' is an anagram indicator.
'about' means one lot of letters goes inside another.
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'sullen' with letters rearranged gives 'lunles'.
'lunles' going around 'u' is 'LUNULES'.

'with' acts as a link.