It's tall, like some Christmas trees (4)
I believe the answer is:
mast
'it's tall' is the definition.
I can't judge whether this definition defines the answer.
'some christmas trees' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'MAST' is hidden within 'christmas trees'.
'like' is the link.
Can you help me to learn more?
(Other definitions for mast that I've seen before include "Upright transmitter; fruit of oak trees" , "Main vertical spar of vessel" , "Long vertical pole" , "Supporting pole, for radio aerial, perhaps" , "One carries the sails of a ship" .)