Speak off the cuff with tips for disguising loans in The Bank (5*)
I believe the answer is:
Here is my best explanation:
'speak off the cuff' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for ad lib that I've seen before include "Off the cuff remark", "Vamp", "extemporise", "Speak unpreparedly", "One's not prepared to do it", "Make an impromptu speech", "Unscripted impromptu remark", "Extemporise", "Speak without planning or preparation" and "with no forethought".)