Student's dog seen with girl regularly (5)
I believe the answer is:
pupil
'student's' is the definition.
('pupil' can be a synonym of 'student')
'dog seen with girl regularly' is the wordplay.
'dog' becomes 'pup' (I have seen 'Young dog ' mean 'pup' so perhaps 'dog' could also mean 'pup'**).
'seen with' says to put letters next to each other.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'girl' are 'il'.
'pup'+'il'='PUPIL'
(Other definitions for pupil that I've seen before include "feature in "Observer"?" , "One studying" , "One being taught" , "The sort of student" , "Dark centre of the eye" .)