Solid girl placed in sink to the left (7)
I believe the answer is:
pyramid
'solid' is the definition.
(pyramid is a kind of solid)
'girl placed in sink to the left' is the wordplay.
'girl' becomes 'mary' (**).
'placed in' means one lot of letters goes inside another (I've seen this in other clues).
'sink' becomes 'dip' ('dip' can be a synonym of 'sink').
'to the left' shows that the letters should be reversed in order (reading from right to left).
'mary' going within 'dip' is 'dimaryp'.
'dimaryp' backwards is 'PYRAMID'.
(Other definitions for pyramid that I've seen before include "Solid figure with triangular sides" , "old monument" , "Object with polygonal base and triangular sides meeting at point" , "Polyhedron, one face a polygon, the rest triangles with common vertex" , "Egyptian landmark" .)