Rocks steamship filled with air (6)
I believe the answer is:
stones
'rocks' is the definition.
('stone' can be a synonym of 'rock')
'steamship filled with air' is the wordplay.
'steamship' becomes 'ss' (nautical abbreviation).
'filled with' means one lot of letters goes inside another.
'air' becomes 'tone' (both can mean something's character or impression).
'ss' placed around 'tone' is 'STONES'.
(Other definitions for stones that I've seen before include "pop group" , "They've been rocking and rolling since the 1960s" , "Attacks" , "Balls" , "Weights and pieces of rock." .)