Printer holds this unit controlled by trustee (5)
I believe the answer is:
toner
'printer holds this' is the definition.
The definition suggests a singular noun which matches the answer.
'unit controlled by trustee' is the wordplay.
'unit' becomes 'one' (synonyms).
'controlled by' is an insertion indicator (some letters restrict or block in others).
'trustee' becomes 'tr' (abbreviation).
'one' inserted within 'tr' is 'TONER'.
Can you help me to learn more?
(Other definitions for toner that I've seen before include "Astringent liquid (to improve skin)" , "Skin-cleansing lotion" , "Lotion for cleansing the skin" , "Powder used in printers" , "Black or coloured powder used in a printer" .)