What’s-his-name’s light adhesive: twelve pence once (11)
I believe the answer is:
thingumabob
'what's-his-name's' is the definition.
(both can be used when you can't remember a name)
'light adhesive twelve pence once' is the wordplay.
'light' becomes 'thin' (similar in meaning).
'adhesive' becomes 'gum' (gum is a kind of adhesive).
'twelve pence once' becomes 'a bob' (a bob or shilling was made up of th pence).
'thin'+'gum'+'abob'='THINGUMABOB'
(Other definitions for thingumabob that I've seen before include "I've forgotten its name" , "unnamed item" , "Doofer" .)