Shade filling each old drain (3,4)
I believe the answer is:
eat into
'drain' is the definition.
(I've seen this in another clue)
'shade filling each old' is the wordplay.
'shade' becomes 'tint' (synonyms).
'filling' is an insertion indicator.
'each' becomes 'ea' (abbreviation).
'old' becomes 'o' (abbreviation).
'ea'+'o'='eao'
'tint' placed inside 'eao' is 'EAT INTO'.
(Other definitions for eat into that I've seen before include "start wearing out" , "Corrode (something)" , "Consume (resources)" , "Etch" , "run down" .)