Breast enhancers I left darling to put back: a mistake (5,4)
I believe the answer is:
false step
'a mistake' is the definition.
(thesaurus)
'breast enhancers i left darling to put back' is the wordplay.
'breast enhancers' becomes 'falsies' (informal term for false breasts).
'left' is a deletion indicator.
'darling' becomes 'pet' (synonyms).
'to put back' shows that the letters should be reversed in order.
'falsies' with 'i' removed is 'falses'.
'pet' in reverse letter order is 'tep'.
'falses'+'tep'='FALSE STEP'
(Another definition for false step that I've seen is " misjudged motion".)