Brendan, say, is prone to break bed, incurring maximum damage (9)
I believe the answer is:
costliest
'incurring maximum damage' is the definition.
'brendan say is prone to break bed' is the wordplay.
'brendan say' becomes 'st' (Saint Brendan one of the 12 apostles of Ireland).
'is prone' becomes 'lies' (I've seen this in another clue).
'to break' is an insertion indicator.
'bed' becomes 'cot' (cot is a kind of bed).
'st'+'lies'='stlies'
'stlies' placed inside 'cot' is 'COSTLIEST'.
(Other definitions for costliest that I've seen before include "most dear" , "Most harmful!" .)