Let oneself down in trouble concealing disease (6)
I believe the answer is:
abseil
'let oneself down' is the definition.
(I've seen this before)
'trouble concealing disease' is the wordplay.
'trouble' becomes 'ail' (synonyms).
'concealing' means one lot of letters goes inside another.
'disease' becomes 'BSE' (bovine spongiform encephalopathy, mad cow disease).
'ail' placed around 'bse' is 'ABSEIL'.
'in' acts as a link.
(Other definitions for abseil that I've seen before include "Use a rope to climb down a mountain" , "Lower oneself with a rope from a mountainside" , "In mountain climbing, descend using a doubled rope" , "Descend a mountain using ropes" , "Descend vertically by rope" .)