Compact, firm (5)
I believe the answer is:
solid
This clue is a double definition.
'compact' is the first definition.
(I've seen this before)
'firm' is the second definition.
(I know that firm can be written as solid)
(Other definitions for solid that I've seen before include "continuous" , "Substantial - having three dimensions" , "Unbroken" , "from one getting pushed around" , "cone-shaped, say" .)