Tablet not finished by serious religious traveller (7)
I believe the answer is:
pilgrim
'religious traveller' is the definition.
(I've seen this before)
'tablet not finished by serious' is the wordplay.
'tablet' becomes 'pill' (synonyms).
'not finished' means to remove the last letter.
'by' says to put letters next to each other.
'serious' becomes 'grim'.
'pill' with its last letter taken away is 'pil'.
'pil'+'grim'='PILGRIM'
(Other definitions for pilgrim that I've seen before include "One of those who sailed to New England in the Mayflower, 1620" , "these early Fathers" , "Person who journeys for religious reasons" , "Travelling worshipper" , "Wanderer" .)