Arachnid runs underneath piano in bank (6)
I believe the answer is:
spider
'arachnid' is the definition.
(I know that spider is a type of arachnid)
'runs underneath piano in bank' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'underneath' says to put letters next to each other (in a down clue, some letters go under others).
'piano' becomes 'p' (musical abbreviation).
'in' is an insertion indicator.
'bank' becomes 'side' (synonyms).
'p' put inside 'side' is 'spide'.
'r' after 'spide' is 'SPIDER'.
(Other definitions for spider that I've seen before include "one with money or house?" , "Web expert?" , "A common arachnid" , "Small creature" , "Creepy-crawly" .)