Learner is in hospital department to sign up (6)
I believe the answer is:
enlist
'sign up' is the definition.
(enlisting is a kind of signing up)
'learner is in hospital department' is the wordplay.
'learner' becomes 'l'.
'in' is an insertion indicator.
'hospital department' becomes 'ent'.
'l'+'is'='lis'
'lis' inserted into 'ent' is 'ENLIST'.
'to' is the link.
(Other definitions for enlist that I've seen before include "Draft" , "augment army" , "Join up - tinsel (anag)" , "Join the military" , "Join the armed forces" .)