Club second to enter sports venue beside lake (7)
I believe the answer is:
arsenal
'club' is the definition.
(I've seen this before)
'second to enter sports venue beside lake' is the wordplay.
'second' becomes 's' (common abbreviation).
'to enter' means one lot of letters goes inside another.
'sports venue' becomes 'arena' (I've seen this in another clue).
'beside' says to put letters next to each other (I've seen this in other clues).
'lake' becomes 'L' (geographical abbreviation).
's' placed into 'arena' is 'arsena'.
'arsena'+'l'='ARSENAL'
(Other definitions for arsenal that I've seen before include "A military store of guns and armaments" , "London premiership football team" , "team effort" , "Military structure for arms and ammunitions" , "London FC" .)