Fat, solidified, covering uniform (4)
I believe the answer is:
suet
'fat' is the definition.
(suet is a kind of fat)
'solidified covering uniform' is the wordplay.
'solidified' becomes 'set' (I've seen this before).
'covering' means one lot of letters goes inside another.
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'set' going around 'u' is 'SUET'.
(Other definitions for suet that I've seen before include "Fat used in foods" , "Thin pancake" , "Hard fat in cattle and sheep" , "Solid fat used in cooking" , "Edible fat used in cooking" .)