Investment of time by one facing deterioration (4,5)
I believe the answer is:
unit trust
'investment' is the definition.
(type of financial investment)
'time by one facing deterioration' is the wordplay.
'time' becomes 't' (abbreviation).
'by' is a charade indicator (letters next to each other).
'one' becomes 'unit' (synonyms).
'facing' means one lot of letters go next to another.
'deterioration' becomes 'rust' (type of deterioration affecting metals).
't' after 'unit' is 'unitt'.
'unitt'+'rust'='UNIT TRUST'
'of' is the link.
(Other definitions for unit trust that I've seen before include "Multiple investment scheme" , "Investment package" , "Investment fund" , "Form of investment" , "Type of investment" .)