Payment of small gratuity on completion (7)
I believe the answer is:
stipend
'payment' is the definition.
(stipend is a kind of payment)
'small gratuity on completion' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'gratuity' becomes 'tip' (as in a gratuity to a waiter).
'on' means one lot of letters go next to another.
'completion' becomes 'end' (I've seen this in another clue).
's'+'tip'+'end'='STIPEND'
'of' acts as a link.
(Other definitions for stipend that I've seen before include "Money paid periodically" , "Regular allowance" , "minister's wages" , "Clergyman's salary" , "Clerical salary" .)