Dangerous reptile descending on American plant (6)
I believe the answer is:
crocus
'plant' is the definition.
The answer is an example.
'dangerous reptile descending on american' is the wordplay.
'dangerous reptile' becomes 'croc' (I've seen this before).
'descending on' says to put letters next to each other (in a down clue, some letters go on top of others).
'american' becomes 'us' (abbreviation for United States).
'croc'+'us'='CROCUS'
(Other definitions for crocus that I've seen before include "Small pretty spring flower, yellow or white" , "Spring bulb" , "it's ready for spring" , "Yellow, purple or white spring flower" , "Bulbous plant with brilliant coloured flowers" .)