Only lean out to catch the ball (5)
I believe the answer is:
alone
'only' is the definition.
(synonyms)
'lean out to catch the ball' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'to catch' is an insertion indicator.
'the ball' becomes 'o' (O looks like a ball).
'lean' with letters rearranged gives 'alne'.
'alne' enclosing 'o' is 'ALONE'.
(Other definitions for alone that I've seen before include "On one's own" , "Without anybody or anything else" , "Without others present" , "unique" , "Without others being involved" .)