Bow legs are awkward, limiting energy for physical effort (5,6)
I believe the answer is:
elbow grease
'physical effort' is the definition.
'elbow grease' can be an answer for 'effort' (synonyms). I'm not certain of the 'physical' bit.
'bow legs are awkward limiting energy' is the wordplay.
'awkward' is an anagram indicator.
'limiting' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'bow'+'legs'+'are'='bowlegsare'
'bowlegsare' with letters rearranged gives 'elbowgrase'.
'elbowgrase' enclosing 'e' is 'ELBOW-GREASE'.
'for' is the link.
Can you help me to learn more?
(Other definitions for elbow grease that I've seen before include "Vigorous physical labour involving hard rubbing" , "Hard rubbing (colloq.)" , "Vigorous physical effort" , "Hard physical effort" , "Hard physical work (informal)" .)