One slipping along to put a gloss finish on the shelf (6)
I believe the answer is:
Here is my best explanation:
'one slipping along to' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'a gloss finish on the shelf' is the wordplay.
'finish' indicates one should take the final letters.
'on' is a charade indicator (letters next to each other).
'the shelf' becomes 'ledge' (synonyms).
The last letter of 'gloss' is 's'.
'put' is the link.