Sampler showing telltale clergyman, one in aisle seat? (5,7)
I believe the answer is:
sneak preview
'sampler' is the definition.
'telltale clergyman one in aisle seat?' is the wordplay.
'telltale' becomes 'sneak' (I've seen this before).
'clergyman' becomes 'rev' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'aisle seat?' becomes 'pew' (seat next to the aisle in a church).
'rev'+'i'='revi'
'revi' put inside 'pew' is 'preview'.
'sneak'+'preview'='SNEAK PREVIEW'
'showing' acts as a link.
(Other definitions for sneak preview that I've seen before include "Unannounced screening" , "Public showing of a film before general release" , "Private screening or viewing of new film etc." , "Test screening before general release" .)