Small head of dandelion, unwanted plant (4)
I believe the answer is:
weed
'unwanted plant' is the definition.
(I've seen this before)
'small head of dandelion' is the wordplay.
'small' becomes 'wee' ('wee' can be similar in meaning to 'small').
'head of' suggests taking the first letters.
The first letter of 'dandelion' is 'd'.
'wee'+'d'='WEED'
(Other definitions for weed that I've seen before include "Plant growing where it is not wanted" , "Wild unwanted plant - tobacco?" , "Intrusive plant; weak person" , "gone!" , "Plant growing in the wrong place!" .)