Certainly expressed twice here to be on the safe side (2,2,4)
I believe the answer is:
to be sure
'certainly' is the definition.
(I've seen this before)
'expressed twice here to be on the safe side' is the wordplay.
I cannot quite see how this works, but
'to' is present in the answer.
'be' is found in the answer.
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for to be sure that I've seen before include "Undoubtedly" , "Of course, certainly" , "Admittedly" , "I admit" , "Without doubt" .)