The DA, for instance, has been known to reduce one's overheads (7)
I believe the answer is:
haircut
'the da for' is the definition.
The definition suggests a singular noun which matches the answer.
'instance has been known to reduce one's overheads' is the wordplay.
I cannot quite see how this works, but
'reduce' could be 'cut' (synonyms) and 'cut' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for haircut that I've seen before include "Crop possibly" , "Bob, say" , "Bob, perhaps" , "DA, for example" , "Bob maybe" .)