Call and help returning student (4)
I believe the answer is:
dial
'call' is the definition.
(thesaurus)
'help returning student' is the wordplay.
'help' becomes 'aid' ('aid' can be a synonym of 'help').
'returning' is a reversal indicator.
'student' becomes 'l' (as in L-plates for learner drivers).
'aid' back-to-front is 'dia'.
'dia'+'l'='DIAL'
'and' acts as a link.
(Other definitions for dial that I've seen before include "Calibrated face" , "Laid up like clock-face" , "Indicator" , "Face of a timepiece" , "Calibrated disc etc" .)