Lose one's cool and pull on plant (10)
I believe the answer is:
snapdragon
'plant' is the definition.
(snapdragon is a kind of plant)
'lose one's cool and pull on' is the wordplay.
'lose one's cool' becomes 'snap' (I've seen this in another clue).
'and' says to put letters next to each other.
'pull' becomes 'drag' (synonyms).
'snap'+'drag'+'on'='SNAPDRAGON'
(Other definitions for snapdragon that I've seen before include "one found in bed" , "Kind of antirrhinum" , "Perhaps Christmas entertainment" , "One blooming" , "one's found in the garden" .)