Diamond ring he oddly mislaid, initially, on coach (7)
I believe the answer is:
rhombus
'diamond' is the definition.
(I've seen this before)
'ring he oddly mislaid initially on coach' is the wordplay.
'initially' indicates taking the first letters.
'on' means one lot of letters go next to another.
'coach' becomes 'bus' ('bus' can be a synonym of 'coach'**).
The first letters of 'ring he oddly mislaid' is 'rhom'.
'rhom'+'bus'='RHOMBUS'
(Other definitions for rhombus that I've seen before include "Equilateral quadrilateral" , "Oblique-angled parallelogram" , "Four-sided figure" , "Quadrilateral with four equal sides" , "4-sided figure" .)