Pigs going outside had tails (7)
I believe the answer is:
shadows
'tails' is the definition.
('shadow' can be a synonym of 'tail')
'pigs going outside had' is the wordplay.
'pigs going' becomes 'sows' (I can't explain this - if you can you should give a lot more credence to this answer).
'outside' indicates putting letters inside.
'sows' placed around 'had' is 'SHADOWS'.
Can you help me to learn more?
(Other definitions for shadows that I've seen before include "They're cast by blocking the sun" , "1960s band" , "Dark things" , "Dogs" , "tracks" .)