Rock fruit over pit (9)
I believe the answer is:
limestone
'rock' is the definition.
(I know that limestone is a type of sedimentary rock)
'fruit over pit' is the wordplay.
'fruit' becomes 'lime' (lime is a kind of fruit).
'over' means one lot of letters go next to another.
'pit' becomes 'stone' (synonyms).
'lime'+'stone'='LIMESTONE'
(Other definitions for limestone that I've seen before include "Sedimentary rock consisting mainly of calcium" , "Rock of calcium carbonate" , "Tie lemons up in sedimentary rock" , "building material" , "Mineral" .)