Angler expending energy unravelling small knots (6)
I believe the answer is:
gnarls
'knots' is the definition.
('gnarl' can be a synonym of 'knot')
'angler expending energy unravelling small' is the wordplay.
'expending' indicates named letters should be taken away.
'energy' becomes 'e' (physics symbol as in E=mc2).
'unravelling' is an anagram indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'angler' with 'e' taken away is 'anglr'.
'anglr' is an anagram of 'gnarl'.
'gnarl'+'s'='GNARLS'
(Another definition for gnarls that I've seen is " Knots in trees".)