Holy man seeing a king hiding in tree (5)
I believe the answer is:
fakir
'holy man' is the definition.
(fakir is a kind of holy man)
'a king hiding in tree' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'hiding in' is an insertion indicator.
'tree' becomes 'fir' (fir is a kind of tree).
'a'+'k'='ak'
'ak' placed within 'fir' is 'FAKIR'.
'seeing' is the link.
(Other definitions for fakir that I've seen before include "Religious mendicant in India" , "Muslim ascetic - Hindu holy man" , "Religious ascetic living solely on alms" , "Eastern holy man" , "Holy man of the East" .)