Hesitation after sending lab blank book (5)
I believe the answer is:
album
'blank book' is the definition.
(I've seen this before)
'hesitation after sending lab' is the wordplay.
'hesitation' becomes 'um' ().
'after' means one lot of letters go next to another.
'sending' indicates anagramming the letters (letters are sent to different positions).
'lab' anagrammed gives 'alb'.
'um' after 'alb' is 'ALBUM'.
(Other definitions for album that I've seen before include "Book for displaying a collection" , "Photograph/stamp book" , "(Photograph) book" , "Pictures" , "Recording; book" .)