Familiar bird, swallow almost caught by coastal mammal (3,4)
I believe the answer is:
sea gull
'familiar bird' is the definition.
(common bird)
'swallow almost caught by coastal mammal' is the wordplay.
'swallow' becomes 'gulp' (gulp is a kind of swallow).
'almost' means to remove the last letter (most of the word but not all of it).
'caught by' indicates putting letters inside.
'coastal mammal' becomes 'seal' (mammal that lives by the coast).
'gulp' with its last letter taken off is 'gul'.
'gul' inserted into 'seal' is 'SEA-GULL'.
(Other definitions for sea gull that I've seen before include "Mostly white aquatic bird" , "Bird paradoxically fond of landfill sites" , "Ullages (anag)" , "Shore bird" , "Marine bird" .)