Mat over hollow surface is wrinkled (6)
I believe the answer is:
rugose
'is wrinkled' is the definition.
(I've seen this before)
'mat over hollow surface' is the wordplay.
'mat' becomes 'rug' (I've seen this before).
'over' becomes 'o' (cricket abbreviation).
'hollow' suggests removing the centre.
'surface' with its middle removed is 'se'.
'rug'+'o'+'se'='RUGOSE'
(Other definitions for rugose that I've seen before include "Wrinkled" , "with sunken lines" .)