White bird's energy mostly increased with time (5)
I believe the answer is:
egret
'white bird's' is the definition.
(I've seen this before)
'energy mostly increased with time' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'mostly' means to remove the last letter.
'increased' becomes 'grew' (growing is a kind of increasing).
'with' is a charade indicator (letters next to each other).
'time' becomes 't'.
'grew' with its final letter taken away is 'gre'.
'e'+'gre'+'t'='EGRET'
(Other definitions for egret that I've seen before include "Lesser white heron" , "White wading bird" , "Water-bird, has plume" , "Mainly white heron" , "One wading" .)