Member beginning to overcome period of uncertainty (5)
I believe the answer is:
limbo
'period of uncertainty' is the definition.
(I've seen this before)
'member beginning to overcome' is the wordplay.
'member' becomes 'limb' (limb is a kind of member**).
'beginning to' suggests taking the first letters.
The first letter of 'overcome' is 'o'.
'limb'+'o'='LIMBO'
(Other definitions for limbo that I've seen before include "State of being disregarded or forgotten" , "Once supposed abode of the souls of unbaptized infants" , "Where things are lost or forgotten or being awaited" , "Uncertain period awaiting a resolution" , "nowhere in particular" .)