Is involved with old firm (5)
I believe the answer is:
solid
'firm' is the definition.
(I know that firm can be written as solid)
'is involved with old' is the wordplay.
'involved with' indicates an anagram (involved can mean confusing or complex).
'is' with its letters anagrammed with 'old' gives 'SOLID'.
(Other definitions for solid that I've seen before include "Rock-hard" , "Three-dimensional - compact" , "Hard and compact" , "Firm - substantial" , "Not hollow or liquid" .)