Page in front of shelf showing undertaking (6)
I believe the answer is:
pledge
'undertaking' is the definition.
(I've seen this before)
'page in front of shelf' is the wordplay.
'page' becomes 'p'.
'in front of' means one lot of letters go next to another (some letters go in front of others).
'shelf' becomes 'ledge' (synonyms).
'p'+'ledge'='PLEDGE'
'showing' acts as a link.
(Other definitions for pledge that I've seen before include "Bond" , "Solemn undertaking" , "Covenant" , "Promise solemnly and formally" , "Promise, oath" .)