Extremely poor climber's secret (5)
I believe the answer is:
privy
'secret' is the definition.
(I know that secret can be written as privy)
'extremely poor climber's' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'climber' becomes 'ivy' (ivy is a climbing plant).
'poor' with its middle taken out is 'pr'.
'pr'+'ivy'='PRIVY'
(Other definitions for privy that I've seen before include "sort of councillor" , "Hidden; outside toilet" , "Party to secret knowledge - outside loo" , "close to the monarch?" , "Sharing in secret knowledge - toilet" .)