Some squirrel babies when climbing are more competent (5)
I believe the answer is:
abler
'more competent' is the definition.
('able' can be similar in meaning to 'competent')
'some squirrel babies when climbing' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'when climbing' says the letters should be written in reverse (in a down clue, letters go up).
'relba' is hidden in the letters of 'squirrel babies'.
'relba' written backwards gives 'ABLER'.
'are' is the link.
(Other definitions for abler that I've seen before include "More proficient" , "Increasingly smart" , "More skilful" , "stronger" , "The better" .)