Deposit's unreasonably small on reflection (4)
I believe the answer is:
soot
'deposit's' is the definition.
(thesaurus)
'unreasonably small on reflection' is the wordplay.
'unreasonably' becomes 'too' (too can mean unreasonably or excessively).
'small' becomes 's' (abbreviation - e.g. clothes size).
'on reflection' says the letters should be written in reverse.
'too'+'s'='toos'
'toos' written backwards gives 'SOOT'.
(Other definitions for soot that I've seen before include "Chimney deposit" , "Stuff from chimneys" , "Fine carbon deposit" , "Carbon black" , "Carbon deposit from burning" .)