Last to show is sent new watch! (7)
I believe the answer is:
witness
'watch' is the definition.
(witnessing is a kind of watching)
'last to show is sent new' is the wordplay.
'last to' says to take the final letters.
'new' indicates anagramming the letters.
The final letter of 'show' is 'w'.
'is'+'sent'='issent'
'issent' with letters rearranged gives 'itness'.
'w'+'itness'='WITNESS'
(Other definitions for witness that I've seen before include "Evidence, proof" , "One sees and reports" , "Testimony - see" , "Onlooker" , "See (an event or crime)" .)