Tissue-type full of fat? (4)
I believe the answer is:
suet
'fat?' is the definition.
(I've seen this before)
'tissue-type full of' is the wordplay.
'full of' indicates a hidden word (inside).
'SUET' is hidden within 'tissue type'.
(Other definitions for suet that I've seen before include "Edible fat used in cooking" , "Solid fat used in cooking" , "Thin pancake" , "Hard white cooking fat" , "Hard animal fat used in cooking" .)