First of Franciscans approaching giant screens, when vespers observed? (9)
I believe the answer is:
nightfall
'when vespers observed?' is the definition.
(vespers is evening prayer)
'first of franciscans approaching giant screens' is the wordplay.
'first of franciscans' becomes 'f' (1st letter of 'franciscans').
'approaching' becomes 'nigh' (as in 'the end is nigh').
'giant' becomes 'tall'.
'screens' indicates putting letters inside (screen can mean to cover up).
'nigh'+'tall'='nightall'
'f' inserted into 'nightall' is 'NIGHTFALL'.
(Other definitions for nightfall that I've seen before include "Dusk" , "Evening" , "gloaming" , "after sunset" , "End of day" .)