Head of martin seen on ailing plant (4)
I believe the answer is:
mill
'plant' is the definition.
(mill is a kind of plant)
'head of martin seen on ailing' is the wordplay.
'head of' says to take the initial letters.
'seen on' is a charade indicator (letters next to each other).
'ailing' becomes 'ill' ('ill' can be similar in meaning to 'ailing').
The initial letter of 'martin' is 'm'.
'm'+'ill'='MILL'
(Other definitions for mill that I've seen before include "Machinery for grinding corn" , "Building in which grain is ground to flour" , "Grinding machine" , "Rob < works" , "Grind (flour)" .)