Frank is the one playing the piano inside (4)
I believe the answer is:
open
'frank' is the definition.
(synonyms)
'the one playing the piano inside' is the wordplay.
'playing' indicates anagramming the letters.
'the piano' becomes 'p' (musical abbreviation).
'inside' indicates putting letters inside.
'one' with letters rearranged gives 'oen'.
'oen' placed around 'p' is 'OPEN'.
'is' is the link.
(Other definitions for open that I've seen before include "begin to > crack" , "Traditional room heater" , "Undecided" , "See 12" , "gaping" .)