Be first, perhaps, to last the person you can most rely on (4,6)
I believe the answer is:
'person you can most rely on' is the definition.
I can't tell whether this defines the answer.
'be first perhaps to last' is the wordplay.
'perhaps' is an anagram indicator.
'to last' becomes 'end' (I've seen this in another clue).
'first' with letters rearranged gives 'stfri'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for best friend that I've seen before include "Mate" , "Bosom companion" .)