Acts regularly to stop plant falling over in wind (6)
I believe the answer is:
breath
'wind' is the definition.
(I know that breath is a type of air)
'acts regularly to stop plant falling over' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'to stop' means one lot of letters goes inside another (stop can mean to plug a hole in something).
'plant' becomes 'herb' (herb is a kind of plant).
'falling over' says the letters should be written in reverse (letters falling back).
The alternate letters of 'acts' are 'at'.
'herb' written backwards gives 'breh'.
'at' put into 'breh' is 'BREATH'.
'in' is the link.
(Other definitions for breath that I've seen before include "Slight breeze; life" , "Air that's inhaled and exhaled" , "Air received and expelled" , "Bather (anag.)" , "An inhalation and exhalation" .)