Plant continually in a rich soil, mostly (4,4)
I believe the answer is:
aloe vera
'plant' is the definition.
The answer is an example.
'continually in a rich soil mostly' is the wordplay.
'continually' becomes 'ever' (ever can mean always).
'in' is an insertion indicator.
'rich soil' becomes 'loam' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'a'+'loam'='aloam'
'aloam' with its last letter taken off is 'aloa'.
'ever' going into 'aloa' is 'ALOE VERA'.
(Other definitions for aloe vera that I've seen before include "Plant juice used in cosmetics" , "Succulent" , "cosmetic juice" , "Plant with juice used for treating burns" , "Mediterranean plant whose thick leaves give off a soothing juice" .)