Firm to rock idols? (5)
I believe the answer is:
solid
'firm' is the definition.
(synonyms)
'rock idols?' is the wordplay.
'rock' indicates an anagram (the solver must 'rock' the letters around).
'idols' with letters rearranged gives 'SOLID'.
'to' acts as a link.
(Other definitions for solid that I've seen before include "Not hollow, the same throughout" , "Firm; not liquid" , "Neither liquid nor gas" , "Reliable - substantial - pure" , "Weighty" .)