Climbing on top of sink, laid a tile (7)
I believe the answer is:
tessera
'tile' is the definition.
(I know that tessera is a type of tile)
'climbing on top of sink laid a' is the wordplay.
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'on top of sink' becomes 'res' (I am not sure about this - if you are sure you should believe this answer much more).
'laid' becomes 'set' (synonyms).
'res'+'set'='resset'
'resset' in reverse letter order is 'tesser'.
'tesser'+'a'='TESSERA'
(Other definitions for tessera that I've seen before include "Bit of design" , "small bit of artwork" , "Piece of picture" , "only part of the picture" , "Small stone for mosaics" .)