Somehow his trees initially gladdened a tourist (9)
I believe the answer is:
sightseer
'a tourist' is the definition.
(sightseer is a kind of tourist)
'somehow his trees initially gladdened' is the wordplay.
'somehow' indicates an anagram.
'initially' says to take the initial letters.
The initial letter of 'gladdened' is 'g'.
'his'+'trees'+'g'='histreesg'
'histreesg' anagrammed gives 'SIGHTSEER'.
(Other definitions for sightseer that I've seen before include "Tripper" , "Visitor to famous locations" , "Tourist visiting scenes of interest" , "Rubberneck" .)