Till you finish, no time in lieu starts (5)
I believe the answer is:
until
'till' is the definition.
(thesaurus)
'you finish no time in lieu starts' is the wordplay.
'finish' suggests the final letters.
'starts' indicates taking the first letters.
The final letter of 'you' is 'u'.
The initial letters of 'no time in lieu' is 'ntil'.
'u'+'ntil'='UNTIL'
(Other definitions for until that I've seen before include "As far as" , "Not beyond (hour)" , "Unlit (anag.)" , "Pending such time as" , "Before a given time" .)