One who loves a busy person has run inside (6)
I believe the answer is:
adorer
'one who loves' is the definition.
(I've seen this in another clue)
'a busy person has run inside' is the wordplay.
'busy person' becomes 'doer' (someone doing things).
'has' is a charade indicator (letters next to each other).
'run' becomes 'r' (cricket abbreviation).
'inside' is an insertion indicator.
'a'+'doer'='adoer'
'adoer' enclosing 'r' is 'ADORER'.
(Other definitions for adorer that I've seen before include "someone in love" , "One loves" , "One would love" , "Someone who loves greatly" , "Lover; worshipper" .)