Problem walking I had, that’s clear (6)
I believe the answer is:
limpid
'clear' is the definition.
('limpid' can be similar in meaning to 'clear')
'problem walking i had' is the wordplay.
'problem walking' becomes 'limp'.
'i had' becomes 'id' (I'd).
'limp'+'id'='LIMPID'
'that's' acts as a link.
(Other definitions for limpid that I've seen before include "Clear - tranquil" , "Crystal clear like pure water" , "Translucent" , "Completely clear" , "Clear, like air or water, say?" .)