Power is rising by three quarters (5)
I believe the answer is:
sinew
'power' is the definition.
(I've seen this before)
'is rising by three quarters' is the wordplay.
'three quarters' means three replacements for 'quarter'.
'rising' is a reversal indicator (going up in a down clue).
'by' means one lot of letters go next to another.
'quarter' becomes 'n'.
'quarter' becomes 'e'.
'quarter' becomes 'w' (**).
'is' back-to-front is 'si'.
'si'+'n'+'e'+'w'='SINEW'
(Other definitions for sinew that I've seen before include "Gristle" , "source of physical strength" , "Muscular strength" , "fibre" , "As synonym for 'strength'" .)