Trees he climbs on a regular basis (4)
I believe the answer is:
elms
'trees' is the definition.
(elm is a kind of tree)
'he climbs on a regular basis' is the wordplay.
'on a regular basis' indicates alternate letters.
'he'+'climbs'='heclimbs'
The alternate letters of 'heclimbs' are 'ELMS'.
(Other definitions for elms that I've seen before include "may have many leaves" , "rugged feature of country churchyard" , "Deciduous trees with rough serrated leaves" , "Trees sound like Cockney steering devices" , "Trees (of late, prone to disease)" .)