Yellowish green colour under tip of such rock (9)
I believe the answer is:
limestone
'rock' is the definition.
(I know that limestone is a type of rock)
'yellowish green colour under tip of such' is the wordplay.
'yellowish green' becomes 'lime' (I've seen this in another clue).
'colour' becomes 'tone' (tone can mean a colour or shade).
'under' says to put letters next to each other.
'tip of' indicates taking the first letters.
The first letter of 'such' is 's'.
'tone' put after 's' is 'stone'.
'lime'+'stone'='LIMESTONE'
(Other definitions for limestone that I've seen before include "Tie lemons up in sedimentary rock" , "Rock of calcium carbonate" , "Caves in here perhaps" , "Basis of cement" , "Clint" .)