Clever — when full of worry is inclined to give up (9)
I believe the answer is:
defeatist
'inclined to give up' is the definition.
'clever when full of worry is' is the wordplay.
'clever' becomes 'deft' (I've seen this before).
'when full of' indicates putting letters inside.
'worry' becomes 'eat' (eating is a kind of worrying).
'eat'+'is'='eatis'
'deft' enclosing 'eatis' is 'DEFEATIST'.
(Other definitions for defeatist that I've seen before include "He does not expect to win" , "He's always expecting to be beaten" , "With this attitude, one is resigned to failure" , "One who always expects failure" , "One giving in too easily" .)