Increasingly dry river being crossed by this stage (9)
I believe the answer is:
thirstier
'increasingly dry' is the definition.
Although both the answer and 'dry' are adjectives, they are not in the same form of the adjective.
'river being crossed by this stage' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'being crossed by' indicates putting letters inside (inserted letters are crossed or covered by others).
'stage' becomes 'tier' (I've seen this in another clue).
'r' going into 'this' is 'thirs'.
'thirs'+'tier'='THIRSTIER'
Can you help me to learn more?
(Another definition for thirstier that I've seen is " needing more water".)