Proceed cautiously, adding a little extra to low digit (6)
I believe the answer is:
tiptoe
'proceed cautiously' is the definition.
(I've seen this in another clue)
'adding a little extra to low digit' is the wordplay.
'adding a little extra' becomes 'tip' (I am not sure about this - if you are sure you should believe this answer much more).
'to low digit' becomes 'toe' (I've seen this before).
'tip'+'toe'='TIPTOE'
Can you help me to learn more?
(Other definitions for tiptoe that I've seen before include "Move carefully" , "Move delicately" , "Walk stealthily" , "Walk very quietly with heels raised" , "Walk very quietly, in song through the tulips" .)