Terrible tool keeps fouling up at first (5)
I believe the answer is:
awful
'terrible' is the definition.
(I know that terrible can be written as awful)
'tool keeps fouling up at first' is the wordplay.
'tool' becomes 'awl' (awl is a kind of tool).
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'at first' suggests taking the first letters.
The initial letters of 'fouling up' is 'fu'.
'awl' enclosing 'fu' is 'AWFUL'.
(Other definitions for awful that I've seen before include "The pits" , "rubbish" , "Very embarrassing" , "gruesome!" , "Fearsome" .)