Block with small room for experiments (4)
I believe the answer is:
slab
'block' is the definition.
(slab is a kind of block)
'with small room for experiments' is the wordplay.
'with' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'room for experiments' becomes 'lab' (I've seen this before).
's' put next to 'lab' is 'SLAB'.
(Other definitions for slab that I've seen before include "Piece, e.g. of cake" , "Chunk of wood" , "Thin flat stone" , "Thick piece of something" , "Flat broad piece as of stone" .)