Small design of lavatory holding gallons (4)
I believe the answer is:
logo
'small design' is the definition.
(I've seen this before)
'lavatory holding gallons' is the wordplay.
'lavatory' becomes 'loo' (loo is a kind of lavatory).
'holding' means one lot of letters goes inside another.
'gallons' becomes 'G' (abbreviation).
'loo' placed around 'g' is 'LOGO'.
'of' is the link.
(Other definitions for logo that I've seen before include "Printed symbol as organisation's emblem" , "A company's printed symbol or emblem" , "Symbolic design" , "Company emblem or device" , "Symbol or emblem used by organisation" .)