Repetitive philippic by leaders in institution, thoroughly execrable (7)
I believe the answer is:
iterant
'repetitive' is the definition.
(I've seen this in another clue)
'philippic by leaders in institution thoroughly execrable' is the wordplay.
'philippic' becomes 'rant' (I've seen this in another clue).
'by' means one lot of letters go next to another.
'leaders in' suggests taking the first letters.
The initial letters of 'institution thoroughly execrable' is 'ite'.
'rant' put after 'ite' is 'ITERANT'.
(Other definitions for iterant that I've seen before include "Echoing" , "Repeating" .)