Work inside cold American plant (6)
I believe the answer is:
cactus
'plant' is the definition.
(I know that cactus is a type of plant)
'work inside cold american' is the wordplay.
'work' becomes 'act' (synonyms).
'inside' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'american' becomes 'us' (abbreviation for United States).
'c'+'us'='cus'
'act' put inside 'cus' is 'CACTUS'.
(Other definitions for cactus that I've seen before include "Thorny desert plant" , "Spiny, succulent plant" , "Prickly desert plant" , "Desert flower" , "Spiny plant of dry regions" .)