Heavy rock they kept loud (5)
I believe the answer is:
hefty
'heavy' is the definition.
(similar in meaning)
'rock they kept loud' is the wordplay.
'rock' indicates anagramming the letters (the solver must 'rock' the letters around).
'kept' is an insertion indicator.
'loud' becomes 'f' (short for forte - 'loud' in music).
'they' with letters rearranged gives 'hety'.
'hety' enclosing 'f' is 'HEFTY'.
(Other definitions for hefty that I've seen before include "With weight" , "Brawny" , "Large, heavy" , "Large and beefy" , "Big and strong" .)