Secretly constrict a gushing European river (5)
I believe the answer is:
tagus
'european river' is the definition.
(I've seen this before)
'secretly constrict a gushing' is the wordplay.
'secretly' says the answer is hidden in the clue.
'TAGUS' is hidden within 'constrict a gushing'.
(Other definitions for tagus that I've seen before include "River of Spain and Portugal" , "River of Spain/Portugal, its mouth at Lisbon" , "Lisbon's river" , "Longest river of Iberian peninsula" , "Longest river of Iberian peninsular" .)