Worthless gas barrel left on top of a hotel (4)
I believe the answer is:
blah
'worthless gas' is the definition.
'blah' can be an answer for 'gas' (thesaurus). I'm not sure about the 'worthless' bit.
'barrel left on top of a hotel' is the wordplay.
'on top of' says to take the initial letters (I've seen 'on top' mean this).
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
The first letters of 'barrel left' is 'bl'.
'bl'+'a'+'h'='BLAH'
Can you help me to learn more?
(Other definitions for blah that I've seen before include "Pretentious nonsense" , "Stupid 20" , "Pretentious words" , "Bombast" , "Dull" .)