Is involved with old firm (5)
I believe the answer is:
'firm' is the definition.
'is involved with old' is the wordplay.
'involved with' indicates anagramming the letters (involved can mean confusing or complex).
'is' anagrammed with 'old' can make '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" .)