Rock hotel in auction (5)
I believe the answer is:
shale
'rock' is the definition.
(shale is a kind of rock)
'hotel in auction' is the wordplay.
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' means one lot of letters goes inside another.
'auction' becomes 'sale' (I've seen this before).
'h' going within 'sale' is 'SHALE'.
(Other definitions for shale that I've seen before include "Stratified rock" , "Oil-bearing rock" , "Layered sedimentary rock" , "Soft rock; heals (anag.)" , "Type of soft rock" .)