Sort of amphibian - little tail and mostly green (2,1,6)
I believe the answer is:
to a degree
'sort of' is the definition.
(thesaurus)
'amphibian little tail and mostly green' is the wordplay.
'amphibian' becomes 'toad' (toad is a kind of amphibian).
'tail' suggests the final letters.
'and' says to put letters next to each other.
'mostly' means to remove the last letter.
The final letter of 'little' is 'e'.
'green' with its final letter taken away is 'gree'.
'toad'+'e'+'gree'='TO A DEGREE'
(Other definitions for to a degree that I've seen before include "in some measure" , "How undergraduate works" , "though not entirely" , "in part" , "Where university course may lead" .)