Colonizer pupil at first learnt about (7)
I believe the answer is:
planter
'colonizer' is the definition.
(thesaurus)
'pupil at first learnt about' is the wordplay.
'at first' suggests taking the first letters.
'about' indicates an anagram.
The first letter of 'pupil' is 'p'.
'learnt' with letters rearranged gives 'lanter'.
'p'+'lanter'='PLANTER'
(Other definitions for planter that I've seen before include "Sowing machine" , "Decorative pot" , "Settler or colonizer" .)