Old Baltic capital's mass independent paper folding (7)
I believe the answer is:
origami
'paper folding' is the definition.
(I've seen this before)
'old baltic capital's mass independent' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'baltic capital' becomes 'riga' (I've seen this before).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'independent' becomes 'i'.
'o'+'riga'+'m'+'i'='ORIGAMI'
(Other definitions for origami that I've seen before include "Artistic paper folding" , "I'm a giro (anag)" , "that's folding" , "Eastern art" , "Creation of objects from paper" .)