Flower in middle of mature border (5)
I believe the answer is:
tulip
'flower' is the definition.
(I've seen this before)
'middle of mature border' is the wordplay.
'middle of' says to take the centre.
'border' becomes 'lip' (I've seen this in another clue).
The central letters of 'mature' are 'tu'.
'tu'+'lip'='TULIP'
'in' is the link.
(Other definitions for tulip that I've seen before include "Spring bulb" , "Flower - lit up (anag)" , "Spring-blooming plant" , "Plant of the lily family with a single showy flower" , "Flower of the Netherlands" .)