Contributions to tutor are reasonably less common (5)
I believe the answer is:
rarer
'less common' is the definition.
('rare' can be the opposite of 'common' so perhaps 'rarer' is reasonable for 'less common')
'contributions to tutor are reasonably' is the wordplay.
'contributions to' says the answer is hidden in the clue (letters contributing to the clue ).
'RARER' is hidden in the letters of 'tutor are reasonably'.
(Other definitions for rarer that I've seen before include "Less often seen" , "even less common" , "Less frequent" , "More scarce or uncommon" , "Thinner" .)