King and priest apt to be dependable (8)
I believe the answer is:
reliable
'be dependable' is the definition.
(I know that dependable can be written as reliable)
'king and priest apt' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'and' means one lot of letters go next to another.
'priest' becomes 'eli' (biblical figure).
'apt' becomes 'able' (I've seen this in another clue).
'r'+'eli'+'able'='RELIABLE'
'to' acts as a link.
(Other definitions for reliable that I've seen before include "Trusty" , "Trustworthy - genuine" , "Dependable, can be trusted" , "worthy of trust" , "Dependable, honest" .)