Dig rock, needing energy (4)
I believe the answer is:
gibe
'dig' is the definition.
(synonyms)
'rock needing energy' is the wordplay.
'rock' becomes 'gib' (both are nicknames for Gibraltar).
'needing' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mci).
'gib'+'e'='GIBE'
(Other definitions for gibe that I've seen before include "A jeering remark" , "Insult" , "Scoff (at) - taunt" , "Hurtful remark" , "Jeer, barb" .)