Small space not available to plant (5)
I believe the answer is:
senna
'plant' is the definition.
The answer is an example.
'small space not available' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'not available' becomes 'na' (abbreviation for not available).
's'+'en'+'na'='SENNA'
'to' is the link.
(Other definitions for senna that I've seen before include "Tropical plant with medicinal leaflets" , "Leaf used as laxative" , "purgative's needed" , "He drove fast" , "taking this to speed up motion?" .)