Lacking some energy, regret entering capital river and lake (10)
I believe the answer is:
serpentine
'lake' is the definition.
(I've seen this before)
'lacking some energy regret entering capital river' is the wordplay.
'lacking' is a deletion indicator.
'some energy' becomes 'e' (physics symbol as in E=mc2).
'regret' becomes 'repent' (synonyms).
'entering' means one lot of letters goes inside another.
'capital river' becomes 'Seine' (river running through Paris).
'repent' with 'e' removed is 'rpent'.
'rpent' going into 'seine' is 'SERPENTINE'.
'and' is the link.
(Other definitions for serpentine that I've seen before include "Snake-like; mineral" , "Dark green mineral" , "water created in London" , "Mineral; like a snake" , "like 21 [SNAKES]" .)