Understand greenhouse effect's content (3)
I believe the answer is:
see
'understand' is the definition.
(synonyms)
'greenhouse effect's content' is the wordplay.
'content' indicates a hidden word (letters contained within the clue).
'SEE' can be found hidden inside 'greenhouse effects'.
(Other definitions for see that I've seen before include "Watch, equal (a bet)" , "Visit" , "Receive" , "Observe a bishopric" , "Diocese of a bishop" .)