Money companion invested in theatre (7)
I believe the answer is:
drachma
'money' is the definition.
(I've seen this before)
'companion invested in theatre' is the wordplay.
'companion' becomes 'ch' (Companion of Honour).
'invested in' means one lot of letters goes inside another.
'theatre' becomes 'drama' (I've seen this before).
'ch' placed within 'drama' is 'DRACHMA'.
(Other definitions for drachma that I've seen before include "Old Greek currency" , "European currency replaced by the euro in 2002" , "ready" , "Old brass" , "money in the past" .)