This writer's wife invested in old money that mustn't be touched (4,4)
I believe the answer is:
live wire
'that mustn't be touched' is the definition.
(touching a live wire is dangerous)
'this writer's wife invested in old money' is the wordplay.
'this writer's' becomes 'ive' ('I have' can refer to the person writing).
'wife' becomes 'w'.
'invested in' is an insertion indicator.
'old money' becomes 'lire' (I've seen this in other clues).
'ive'+'w'='ivew'
'ivew' inserted inside 'lire' is 'LIVE WIRE'.
(Other definitions for live wire that I've seen before include "dynamic chap" , "isn't it shocking?" , "Alert and enthusiastic person" , "Very sparky person" , "Life and soul of party" .)