Somewhat lacklustre, a clergyman's sentimentality (7)
I believe the answer is:
treacle
'sentimentality' is the definition.
(treacle can mean sentimental speech)
'somewhat lacklustre a clergyman's' is the wordplay.
'somewhat' indicates the answer is hidden within the clue.
'TREACLE' is hidden in the letters of 'lacklustre a clergymans'.
(Other definitions for treacle that I've seen before include "Sweet sticky syrup" , "cloying stuff" , "Sweet, black viscous substance" , "flattery" , "Dark viscous 11" .)