Hard to explain at first, pager or an alarm going off (10)
I believe the answer is:
paranormal
'hard to explain' is the definition.
(not explained by science)
'first pager or an alarm going off' is the wordplay.
'first pager' becomes 'p' (1st letter of 'pager').
'going off' is an anagram indicator.
'or'+'an'+'alarm'='oranalarm'
'oranalarm' with letters rearranged gives 'aranormal'.
'p'+'aranormal'='PARANORMAL'
'at' acts as a link.
(Other definitions for paranormal that I've seen before include "Scientifically inexplicable" , "mystic" , "Beyond scientific understanding" , "on a higher plane?" , "Supernatural" .)