Only outside the top sheds and secretly (2,3,3)
I believe the answer is:
on the sly
'secretly' is the definition.
(synonyms)
'only outside the top sheds' is the wordplay.
'outside' means one lot of letters goes inside another (some letters go outside others).
'top' suggests taking the first letters (in a down clue, the first letter is top).
The initial letter of 'sheds' is 's'.
'the'+'s'='thes'
'only' placed around 'thes' is 'ON THE SLY'.
'and' acts as a link.
(Other definitions for on the sly that I've seen before include "Behind someone's back" , "under cover" , "Surreptitiously" , "With craft" , "Secretly or furtively" .)