Tailless parasite covering head of tulip plant (5)
I believe the answer is:
lotus
'plant' is the definition.
(lotus is a kind of plant)
'tailless parasite covering head of tulip' is the wordplay.
'tailless' means to remove the last letter.
'parasite' becomes 'louse' (I've seen this before).
'covering' indicates putting letters inside.
'head of' says to take the initial letters.
The first letter of 'tulip' is 't'.
'louse' with its final letter removed is 'lous'.
'lous' enclosing 't' is 'LOTUS'.
(Other definitions for lotus that I've seen before include "Plant promoting forgetfulness" , "In Greek mythology, a fruit which induces forgetfulness" , "Sacred waterlily" , "Water-lily; yoga position" , "----- Eaters, indolent hedonists" .)