Release Papa's brother - like Papa, lacking energy (7)
I believe the answer is:
unclasp
'release' is the definition.
(I've seen this before)
'papa's brother like papa lacking energy' is the wordplay.
'papa's brother' becomes 'uncle'.
'like' becomes 'as'.
'papa' becomes 'p' (phonetic alphabet: alpha, bravo, charlie etc.).
'lacking' is a deletion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'uncle'+'as'+'p'='uncleasp'
'uncleasp' with 'e' taken away is 'UNCLASP'.
(Other definitions for unclasp that I've seen before include "let go" , "Unfasten" , "Detach (necklace, belt, etc)" , "Loosen grip on" , "Open - release from grip" .)